Francois Hollande lays a wreath at the Tomb of the Unknown Soldier at the Arc de Triomphe in Paris on Friday, during a ceremony to mark 70 years since victory over Nazi Germany during World War II.

AFP/Paris

Europe held sombre ceremonies to mark 70 years since victory over Nazi Germany on Friday as leaders warned of modern day threats such as the war in Ukraine and Islamic extremism.

Celebrations of the WWII victory in Europe were muted a day before Moscow rolls out its full military might at a parade which is being snubbed by Western leaders due to tensions over the crisis in Ukraine.

Poland opened Victory Day celebrations with a midnight ceremony in northern Westerplatte, where the first shots of the war were fired on September 1, 1939 as Nazi forces swept across the border.

In France, President Francois Hollande laid a wreath at the Tomb of the Unknown Soldier at a solemn ceremony under the Arc de Triomphe in Paris after urging citizens not to grow complacent about war.

"We didn't experience the war, we see it as a far-off reality, sometimes abstract, even though it is not so far from us, in Ukraine, further still in the Middle East," he said.

Hollande also referred to the hundreds of French citizens who have gone to fight alongside jihadists in Syria and Iraq.

"There is also terrorism which can strike us, racism, anti-Semitism. There are still causes which should spur us on."

France is still recovering from a jihadist killing spree in and around Paris in January which left 17 people dead over three days.

"I think in the wake of what happened in January we must come together, find common ground," said Jean Ruiz, witnessing the ceremony on the Champs Elysees avenue.

"There are memories which must not be forgotten," said his wife Mireille.

Allied troops 'saved world'

US President Barack Obama also marked the anniversary, expressing solidarity with Europe and hailing the victorious Allied troops as a "generation that literally saved the world."

"For over five years, brutal fighting laid waste to an entire continent. Mothers, fathers, children were murdered in concentration camps. By the time the guns fell silent in Europe, some 40mn people on the continent had lost their lives," Obama said in a radio address.

In Washington DC, 56 restored American "warbirds" that helped the Allies achieve victory in the war, will roar over the city later Friday as up to 10,000 people look on.

US Secretary of State John Kerry, whose mother was born in France, also laid a wreath under the Arc de Triomphe as the anniversary coincided with a visit to Paris.