US President Barack Obama has paid tribute to the men who breached "Hitler's Wall" and stormed ashore on D-Day in June 1944.

He said their sacrifice bought a still-evolving age of democracy and freedom.

Mr Obama was speaking at a ceremony in northern France to mark the 70th anniversary of the Allied invasion.

"By the end of that longest day, this beach had been fought, lost, refought and won - a piece of Europe once again liberated and free.

"Hitler's Wall was breached, letting loose Patton's Army to pour into France," Mr Obama said at the US war cemetery at Omaha Beach.

The main D-Day ceremony is taking place on Sword Beach in Normandy today. 

French President Francois Hollande opened the ceremonies, paying homage to civilians and soldiers who lost their lives on the day "that changed the world".

"This day, which began in chaos and fire, would end in blood and tears, tears and pain, tears and joy at the end of 24 hours that changed the world and forever marked Normandy," President Hollande said. 

Elsewhere, British Prime Minister David Cameron said he felt a mixture of "awe and gratitude" as he met veterans of the D-Day landings.

Mr Cameron said it was "incredibly moving" to be at the events in Normandy and it was "humbling" for people of his generation.

With Russian president Vladimir Putin's presence highlighting current divisions in Europe over Ukraine, Mr Cameron stressed the role played by Russia in liberating the continent from Nazi tyranny.

"I think the clear evidence of what happened in 1944 and 1945 is the importance of standing up together for freedom and security," he said in Bayeux.

"And we should remember that, and the importance of NATO and thinking forward to the NATO summit in Wales in September.

"But I think it's right today, of all days, to remember all those who served and all those who died.

"Yes, of course we have our disagreements today with Russia, but we should never forget that Russia - the Soviet Union - was an ally of Britain and America, the Free French, Canadian and Australian forces, that liberated this continent from the tyranny of Nazism."

US President Dwight D. Eisenhower sent this letter 5 June 1944, just hours prior to the D-day landings in Normandy. 

Eisenhower's letter