World leaders lined up to pay tribute to Joe Biden after he announced Sunday he was dropping out of the 2024 presidential race.

Biden notified the public of his decision in a letter, a stunning move that upends the race for the White House. He endorsed Vice President Kamala Harris as the Democratic Party's new nominee.

Here's how other world leaders responded to the bombshell news.

Ukrainian leader Volodymyr Zelensky thanked Biden for taking "bold steps" in supporting his country, praising the US president's "tough but strong decision" to end his reelection bid.

"He supported our country during the most dramatic moment in history, assisted us in preventing (Russian President Vladimir) Putin from occupying our country, and has continued to support us throughout this terrible war," he said.

Polish Prime Minister Donald Tusk praised Biden for taking "many difficult decisions thanks to which Poland, America and the world are safer, and democracy stronger."

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"I know you were driven by the same motivations when announcing your final decision. Probably the most difficult one in your life," added Tusk, who served as the European Council president between 2014 and 2019.

UK Prime Minister Keir Starmer said he respected Biden's decision.

"I know that, as he has done throughout his remarkable career, he will have made his decision based on what he believes is best for the American people," said Starmer.

German Chancellor Olaf Scholz also paid tribute to "my friend."

"Joe Biden has achieved a lot: for his country, for Europe, for the world," he said. "His decision not to run again deserves respect."

Israel's Defense Minister Yoav Gallant thanked Biden for his "unwavering support of Israel over the years."

In reference to Israel's brutal assault on Gaza, he added: "Your steadfast backing, especially during the war, has been invaluable. We are grateful for your leadership and friendship."

Japan's premier said he recognized that Biden's announcement is one "he feels is the best political decision he can make."

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"The Japan-US alliance is the cornerstone of our country's diplomacy and security, so we will monitor the situation closely," Prime Minister Fumio Kishida said.

Australian Prime Minister Anthony Albanese thanked Biden "for your leadership and ongoing service."

"The Australia-US alliance has never been stronger with our shared commitment to democratic values, international security, economic prosperity and climate action for this and future generations."

Venezuela's President Nicolas Maduro told Biden "to be at ease with your conscience because you took the sensible and correct decision."

"He prioritized his family, his health and he realized at that age, with failing health, he couldn't take the reins of his country, much less a presidential candidacy," said Maduro, who is preparing for presidential polls this month.

Canadian Prime Minister Justin Trudeau also thanked Biden for his years of service. "He's a great man, and everything he does is guided by his love for his country," he said.

"As president, he is a partner to Canadians – and a true friend. To President Biden and the First Lady: thank you."

Former president Barack Obama, with whom Biden served two terms as vice president, praised his record in office. "Internationally, he restored America's standing in the world, revitalized NATO, and mobilized the world to stand up against Russian aggression in Ukraine," he said.

While he had every right to run for reelection, Biden's decision to drop out of the race was testament to his "love of country," Obama added.

The Kremlin said it was monitoring developments.

"The election is still four months away. And it's a long time, during which a lot can change. We need to pay attention, follow what will happen and go about our business," Kremlin spokesperson Dmitry Peskov told the Life.ru news outlet.

Even as world leaders paid tribute to his performance on the world stage, leading Republicans were insisting he was unfit to remain president.

"If Joe Biden is not fit to run for president, he is not fit to serve as president," said a statement from House Speaker Mike Johnson, the top Republican in Congress.

"He must resign the office immediately. November 5 cannot arrive soon enough," he added.

Former president Donald Trump, the 2024 Republican nominee, wrote on his Truth Social network: "Crooked Joe Biden was not fit to run for president, and is certainly not fit to serve."