U.S. President Joe Biden on Thursday warned on the 80th anniversary of D-Day that democracy around the world was at risk, as leaders marked the 1944 landings in occupied France that helped defeat Nazi Germany in World War II.

Haunted by today's war in Europe following Russia's invasion of Ukraine, Biden, Britain's King Charles III, Canadian Prime Minister Justin Trudeau and French President Emmanuel Macron paid tribute to the tens of thousands of Allied troops who stormed the beaches of Normandy in northern France on June 6, 1944.

The commemorations also provided a hugely symbolic backdrop to talks on how Kyiv can regain ground after Russian advances, with Ukrainian President Volodymyr Zelensky attending an international ceremony with all the leaders.

Biden said D-Day showed the need for international alliances and vowed never to abandon Ukraine in its fight against Russia, in a pointed swipe at his election rival ex-president Donald Trump who has publicly questioned the importance of organisations such as NATO.

"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," Biden said.

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

"Real alliances make us stronger -- a lesson that I pray we Americans never forget."

'Will not end there'

Biden also vowed that, under his leadership, the United States "will not walk away" from Ukraine "because if we do, Ukraine will be subjugated and it will not end there".

"Ukraine's neighbours will be threatened, all of Europe will be threatened," he added, describing Russian President Vladimir Putin as a "tyrant bent on domination".

He said that to surrender to bullies or "bow down to dictators" is "simply unthinkable".

Kyiv has been pushing Europe to up its military support, with Russia in recent months gaining the upper hand on the battlefield and concerns growing over what a Trump presidency could mean for the conflict.

"Allies defended Europe's freedom then, and Ukrainians do so now. Unity prevailed then, and true unity can prevail today," Zelensky said in a post on X.

Arriving at the ceremony he shook hands with wheelchair-bound veterans, bowing down in respect to them.

'Oppose tyranny'

Canada's Trudeau said democracy was "threatened by aggressors who want to redraw borders".

"Our way of life did not happen by accident," he said at the Canadian ceremony.

The biggest guests of honour were some 180 surviving veterans in their late 90s or even over 100, some in wheelchairs and huddled in blankets as they gazed over the shores.

Macron awarded a dozen of them France's highest honour, the Legion d'Honneur.

King Charles III at the British memorial at Ver-sur-Mer that overlooks Gold beach, one of the landing sites for British troops, said: "Our gratitude is unfailing and our admiration eternal."

"Free nations must stand together to oppose tyranny," he said. "Let us pray such sacrifice need never be made again."

'Can never diminish'

British veteran Cecil Newton said he was upset that so many young men were killed.

"All those who were in action will always be with me, with us. I can see them now," Newton, 100, said.

King Charles noted that veterans were becoming fewer, adding: "Our obligation to remember them, what they stood for and what they achieved for us all can never diminish."

Two veterans were unable to make it to France.

William Cameron, a 100-year-old Canadian veteran, had packed his bags weeks in advance but died just before he was to return to France, while 102-year-old Robert Persichitti from the United States died on a ship en route to the ceremony.

Ceremonies were also sprinkled with stardust with film director Steven Spielberg and actor Tom Hanks present in acknowledgement of their classic 1998 film "Saving Private Ryan" about D-Day.

For the British ceremony, veteran crooner Tom Jones and popular folk singer and actor Johnny Flynn provided music.

No Russian official has been invited, underlining Moscow's current pariah status despite the decisive Soviet contribution to defeating Nazism in World War II.

During a meeting with foreign news outlets in Saint Petersburg late Wednesday, Putin shrugged off the lack of an invitation for Russia, saying "let them celebrate without us".

© Agence France-Presse