US President Joe Biden warned that democracy is once again under threat in a speech to commemorate D-Day in Normandy, France, on Thursday.

"We're living in a time when democracy is more at risk across the world than at any point since the end of World War II," he said at the American Cemetery in Colleville-sur-Mer.

"Isolationism was not the answer 80 years ago and is not the answer today."

D-Day was the largest amphibious assault in history. It laid the foundations for the Allied victory over Nazi Germany in World War II, with Biden calling it a "powerful illustration of how alliances, real alliances make us stronger."

Britain's King Charles III and Canadian Prime Minister Justin Trudeau, representing the major Allied powers that participated in the military operation, also attended the memorials in Normandy.

"We recall the lesson that comes to us, again and again, across the decades: free nations must stand together to oppose tyranny," Charles said.

They were joined by around 200 surviving veterans.

Zelenskyy receives standing ovation

Ukrainian President Volodymyr Zelenskyy received a standing ovation from leaders and other attendees as he arrived at the memorial on Omaha Beach with his wife Olena Zelenska.

"Thank you to the Ukrainian people for their bravery. We are here and we will not weaken," French President Emmanuel Macron said.

"Faced with the return of war on our continent ... faced with those who purport to change borders by force to re-write history, let us be worthy of those who landed here," he added.

Russia was not invited to take part in the event despite the Soviet contribution to defeating Nazi Germany — an absence that highlighted Moscow's status as an international pariah.

Earlier in the day, Biden used the opportunity to underscore Washington's support for Ukraine in the face of Russian aggression.

"We will not walk away because if we do, Ukraine will be subjugated, and it will not end there," Biden said.

"Ukraine's neighbors will be threatened, all of Europe will be threatened."

Biden described Russian President Vladimir Putin as a "tyrant bent on domination."

"To surrender to bullies, to bow down to dictators, is simply unthinkable," he added.

"If we were to do that, it means we'd be forgetting what happened here on these hallowed beaches."

European leaders thank veterans

World leaders attended several other events held on both sides of the English Channel to honor those who sacrificed their lives 80 years ago.

At a ceremony for Canadian soldiers on Juno Beach in Normandy, French Prime Minister Gabriel Attal said his country's gratitude was "eternal."

"France will never forget those who died for her," he said.

Meanwhile, Macron awarded 11 US veterans and one British veteran with France's highest distinction, the Legion of Honor.

"You are back here today at home, if I may say," he said in English to the veterans, who are now aged between 98 and 104 years old.

German Chancellor Olaf Scholz said the soldiers' courage paved the way for freedom and democracy in his own country.

"Their courage gave us Germans the chance of a new beginning," he wrote in an article published in the Ouest France newspaper.

zc/nm (AP, Reuters, AFP)