London (CNN)Joe Biden was sworn in Wednesday as the 46th United States President, taking the reins of a country in crisis. But his task on the global stage will be daunting, too.
World leaders reacted to Biden's inauguration by offering congratulations, jockeying for position at the forefront of his foreign policy agenda, and in some cases pleading for the reversal of his predecessor's policies.
Among most messages was a palpable sense of relief, as the international community embraced Biden's pledge to reenter a series of global pacts and organizations that President Donald Trump cut loose.
Here's what leaders have said so far.
"Once again, after four long years, Europe has a friend in the White House," European Commission President Ursula von der Leyen said on Wednesday, leaving no uncertainty about her verdict on Trump's relationship with the bloc.
"This new dawn in America is the moment we've been waiting for so long. Europe is ready for a new start with our oldest and most trusted partner," she said in the European Parliament in Brussels, Belgium.
Von der Leyen said Biden's inauguration would "be a message of healing for a deeply divided nation and it will be a message of hope for a world that is waiting for the US to be back in the circle of like-minded states."
On Twitter, she added: "The United States is back. And Europe stands ready. To reconnect with an old and trusted partner, to breathe new life into our cherished alliance."
Biden has signaled a warmer partnership with Europe than Trump, who frequently criticized the EU on trade during his administration. His attacks on some European leaders led to frosty scenes at a number of summits.
"From our perspective, Trump saw Europe as an enemy," a senior European diplomat told CNN last week. "The lasting impact of 'America First' is the US having fewer friends in Europe."
German Chancellor Angela Merkel also offered her "warmest congratulations," adding that she looks forward to a "new chapter" in Germany's relationship with the US, according to a tweet from her spokesman Steffen Seibert.
And Germany's President expressed relief that Biden would be sworn in Wednesday, calling it "a good day for democracy." "In the United States, (democracy) held up against a lot of pressure," Frank Walter Steinmeier said in a statement. "Despite internal hostility, America's institutions have proven strong -- election workers, governors, judiciary, and Congress. I am relieved that Joe Biden is sworn in as President today and coming into the White House. I know that this feeling is shared by many people in Germany."
Steinmeier also warned against the populist brand of politics that Trump embraced. "Despite all the joy we have about today, we must not forget that populism has seduced even the most powerful democracy in the world," he said. "We must resolutely oppose polarization, protect and strengthen the public space of our democracies, and shape politics on the basis of reason and facts."
French President Emmanuel Macron marked the day with a tweet welcoming the US back into the Paris climate agreement. "We are together. We will be stronger to face the challenges of our time. Stronger to build our future. Stronger to protect our planet. Welcome back to the Paris Agreement," Macron said.
Biden will sign a slate of executive actions in the Oval Office on Wednesday, including one to rejoin the Paris Agreement.
Hours before the inauguration, Beijing expressed hope that Biden would "look at China rationally and objectively" to repair "serious damage" in bilateral ties caused by the Trump presidency.
"In the past four years, the US administration has made fundamental mistakes in its strategic perception of China ... interfering in China's internal affairs, suppressing and smearing China, and causing serious damage to China-US relations," foreign ministry spokeswoman Hua Chunying said at a press briefing Wednesday.
The Biden administration should, Hua said, "look at China rationally and objectively, meet China halfway and, in the spirit of mutual respect, equality and mutual benefit, push China-US relations back to the right track of healthy and stable development as soon as possible."
One of the main planks of Trump's foreign policy platform has been his trade war with China. The Trump administration's 11th-hour declaration that China is committing genocide against Uyghur Muslims will heighten tensions with Beijing, though Biden's nominee for Secretary of State said Tuesday that he agreed with the designation.
"If the new US administration can adopt a more rational and responsible attitude in formulating its foreign policy, I think it will be warmly welcomed by everyone in the international community," she added.
Russia said it hoped for a "more constructive" relationship between the US and Moscow as Biden took office.
The Russian Foreign Ministry said in a statement on Wednesday that it wanted to extend the landmark Strategic Arms Reduction Treaty (New START) with the United States for a maximum stipulated five-year period.
The US-Russia arms treaty is currently due to expire just 16 days after Inauguration day.
"We consider it possible to prolong it only in the form in which the Agreement was signed and without any preconditions. An extension for the maximum five-year term stipulated in the Agreement looks preferable," the Foreign Ministry said in a statement released just after Biden was sworn in.
The statement added that Moscow hoped for a "a more constructive approach in the dialogue with Russia and will take into account all the points mentioned above."
It concluded: "For our part, we are ready for such work on the principles of equality and mutual consideration of interests," the statement concluded.
Biden has previously signaled that he intends to take a tough line against potential Russian aggression.
Antony Blinken, Biden's pick to lead the State Department, said Tuesday that Biden intends to seek an extension of the New START Treaty, but suggested he has not made a decision on the length of the extension.
Iran's President, Hassan Rouhani, called on Biden to return to the 2015 nuclear deal and lift US sanctions on Iran, overturning a key part of Trump's foreign policy program.
"The ball is in the US' court now. If Washington returns to Iran's 2015 nuclear deal, we will also fully respect our commitments under the pact," Rouhani said in a televised cabinet meeting.
He also launched a scathing attack on the outgoing President. A "tyrant's era came to an end and today is the final day of his ominous reign," Rouhani said of Trump's departure. "Someone for whom all of his four years bore no fruit other than injustice and corruption and causing problems for his own people and the world."
Biden has said he plans to return to the nuclear deal with Iran, which was signed when he was Barack Obama's Vice President. Biden's national security aides have suggested they would like further negotiations on Iran's ballistic missile capabilities, though Rouhani has said the missile program is non-negotiable.
Iran's Foreign Minister Mohammad Javad Zarif notably did not congratulate President Biden or Vice President Harris. Zarif did say that he hoped the pair could learn from what he described as the Trump administration's mistakes.
"Trump, Pompeo & Co. are relegated to the dustbin of history in disgrace," Zarif said on Twitter Wednesday, shortly after the inauguration ceremony. "Perhaps new folks in DC have learned."
Elsewhere in the region Israel's Prime Minister Benjamin Netanyahu congratulated Biden and Harris on what he called their "historic inauguration," and reminded the new US leader he sees Iran as their number one challenge to confront.
"President Biden, you and I have had a warm personal friendship going back many decades. I look forward to working with you to further strengthen the US-Israel alliance, to continue expanding peace between Israel and the Arab world and to confront common challenges, chief among them the threat posed by Iran," Netanyahu said in a video statement.
Palestinian Authority President Mahmoud Abbas also congratulated the pair Wednesday and stressed his willingness to continue with the peace process.
"We look forward to working together for peace and stability in the region and the world," he said in a letter to Biden, the official Palestinian news agency Wafa reported.
In the letter, Wafa reports, Abbas "affirmed his readiness for 'a comprehensive and just peace process that would achieve the aspirations of the Palestinian people in freedom and independence.'"
The Palestinian Authority broke political ties with the United States in December 2017 after then-President Trump recognized Jerusalem as the capital of Israel, with President Abbas declaring the US could no longer be an honest broker in peace talks.
Jordan's King Abdullah II also welcomed Biden to the world stage.
"Warmest congratulations to [Joe Biden] on his inauguration today. We highly value our strategic partnership and enduring friendship with the United States, and we look forward to working with you in pursuit of global peace and prosperity," the monarch said on Twitter.
British Prime Minister Boris Johnson said he was looking forward to a close relationship with Biden's administration.
"In our fight against Covid and across climate change, defence, security and in promoting and defending democracy, our goals are the same and our nations will work hand in hand to achieve them," Johnson said in a statement Tuesday.
Johnson warmly welcomed Trump on his visits to the UK, with Trump once claiming that the Prime Minister was nicknamed "Britain Trump." But the outgoing US leader was unpopular among Britons, and Johnson will be keen to secure a post-Brexit trade deal with Biden.
The new President could end up making two trips to the UK in 2021, with Johnson saying he looks forward to welcoming him to the G7 summit and to the hotly anticipated 2021 United Nations Climate Change Conference in Glasgow, Scotland. "Only through international cooperation can we truly overcome the shared challenges which we face," Johnson said on Tuesday.
Scotland's First Minister, Nicola Sturgeon, was more blunt in her remarks in the Scottish Parliament on Wednesday. Sturgeon wished Biden and Kamala Harris well, adding: "I'm sure many of us across the chamber and across Scotland will be very happy to say cheerio to Donald Trump today."
"I think 'don't haste ye back' might be the perfect rejoinder to him," she added.
Queen Elizabeth also sent a private congratulatory message ahead of the inauguration to Biden, a royal source said Wednesday.
The letter's content was not disclosed, but the move does follow precedent. The monarch typically congratulates fellow heads of state at their inauguration.
Shortly after Biden swearing-in, Canadian leader Justin Trudeau said he would work with the new US President "to make our countries safer, more prosperous, and more resilient."
"Canada and the United States enjoy one of the most unique relationships in the world, built on a shared commitment to democratic values, common interests, and strong economic and security ties," he said. "Our two countries are more than neighbours -- we are close friends, partners, and allies.
"Canada and the United States have worked side-by-side to tackle some of the greatest challenges we have faced in our history," Trudeau also said.
Mexico's President, Andrés Manuel López Obrador, wished Biden well ahead of his inauguration on Wednesday.
During his daily briefing, López Obrador outlined three themes as key areas of the bilateral relationship with the US. "Those three themes are very important: pandemic, economic recovery and migration," he said.
López Obrador also said Biden should take steps to settle the immigration status of Mexicans working in the US.
Brazil's President Jair Bolsonaro, widely considered a Trump ally, took to Twitter to congratulate the incoming US leader.
"I salute Joe Biden as the 46th President of the USA," Bolsonaro wrote.
"The relationship [between Brazil and the US] is long, solid and based on higher values such as the defense of democracy and individual liberties. I continue committed and ready to work towards the prosperity of our nations and the well-being of our citizens."
Bolsonaro said he had written Biden a letter "congratulating him for his inauguration and explaining my vision for an excellent future for the Brazil-US relationship."
Ecuador's leader Lenin Moreno was particularly enthusiastic in his well wishes.
"It is a great day for the United States," he wrote on Twitter. "Democracy triumphs, respect for the will of the people prevails, and those principles are strengthened in all the nations of the region."
The leaders of Peru, Colombia, Argentina, Paraguay and Bolivia also congratulated Biden and Harris.
The Vatican published Pope Francis' message to President Joe Biden, the United States' second Catholic President, following his inauguration.
"On the occasion of your inauguration as the forty-sixth President of the United States of America, I extend cordial good wishes and the assurance of my prayers that Almighty God will grant you wisdom and strength in the exercise of your high office," the message says.
"Under your leadership, may the American people continue to draw strength from the lofty political, ethical and religious values that have inspired the nation since its founding.
"At a time when the grave crises facing our human family call for farsighted and united responses, I pray that your decisions will be guided by a concern for building a society marked by authentic justice and freedom, together with unfailing respect for the rights and dignity of every person, especially the poor, the vulnerable and those who have no voice," the Pope wrote.
In a series of tweets, Indian Prime Minister Narendra Modi offered his "warmest congratulations" to US President Joe Biden. Modi said he looks forward to working with Biden and "to strengthen (the) India-US strategic partnership."
"The India-US partnership is based on shared values," he wrote. "We have a substantial and multifaceted bilateral agenda, growing economic engagement and vibrant people to people linkages. Committed to working with President @JoeBiden to take the India-US partnership to even greater heights."
NATO's Secretary General Jens Stoltenberg called Biden's inauguration "the start of a new chapter for the transatlantic Alliance."
"U.S. leadership remains essential as we work together to protect our democracies, our values and the rules-based international order," he said.
"NATO Allies need to stand together to address the security consequences of the rise of China, the threat of terrorism, including in Afghanistan and Iraq, and a more assertive Russia."
CNN's Ramin Mostaghim, Eleanor Pickston, Luke McGee and Stephanie Halasz contributed.