Normandy, France - 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 bring about victory against Nazi Germany in World War II.

Alongside French President Emmanuel Macron, Biden paid tribute to the tens of thousands of Allied troops who landed on the sandy beaches of Normandy in northern France on June 6, 1944.

The 81-year-old D-Day showed the need for international alliances, in a pointed swipe at Donald Trump, who has publicly bashed organization 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.

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"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."

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

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

The biggest guests of honor 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.

Biden individually met 41 US veterans of the Normandy campaign, of whom 33 were present on D-Day itself.

Macron awarded some of them with France's highest honor, the Legion d'Honneur, with many struggling up from their wheelchairs to receive the medal.

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Ceremonies were also sprinkled with stardust, with film director Steven Spielberg and actor Tom Hanks present in acknowledgment of their classic 1998 film, Saving Private Ryan.

The event provided a hugely symbolic backdrop to talks on how Ukraine can regain ground after Russian advances, with President Volodymyr Zelensky in Normandy to attend an international ceremony with all the leaders.

Conversely, it also took place amid Israel's relentless war on Palestinians in Gaza and the West Bank, an assault facilitated by US weapons and diplomatic cover, and in defiance of post-war international norms and institutions.