French President Francois Hollande has urged people to fight global threats to peace with the "same vision, the same courage" as those who fought on D-Day, BBC News reports.
Almost 2,000 veterans joined world leaders at a ceremony in Normandy, France, marking the 70th anniversary of the momentous World War Two mission.
Hollande said today's threats included terrorism, global warming and mass unemployment. Earlier, the Queen laid a wreath at a military cemetery in Bayeux.
The main commemoration event took place at Sword Beach, one of five landing points for the Allies, where scenes from the 1944 invasion were re-enacted.
The landings - involving 156,000 troops - were the first stage of the invasion of Nazi-occupied Europe. By the end of D-Day on 6 June 1944, the Allies had established a foothold in France - an event that would eventually help bring the war to an end. More than 4,000 British, American and Canadian troops lost their lives on that first day of the battle.
The leaders flew in to draw political lessons, seeking to define D-Day in their own ideological terms.
President Obama, speaking at a war cemetery that is the last resting place for nearly 10,000 fallen soldiers, spoke of a sacrifice in the service of "our commitment to liberty, to equality, to freedom, to the inherent dignity of every human being".
Hollande said: "It's up to us to have the same vision, the same courage, to be just as bright and have the same determination as those who came to these beaches 70 years ago."
Hollande also paid tribute to "the courage of all these young men who came from all over the world to conquer - meter after meter, inch after inch - the beaches and the dunes".
"I really wish to pay tribute to the courage and the Germans who were actually victims of Nazi rule. They were led into a war which was not theirs and which would have never been theirs. And today we really want to pay homage to all the victims of Nazi rule."
Hollande also called for the beaches of Normandy to become a UNESCO World Heritage site.
A 21-gun salute and a flypast also formed part of proceedings at the Sword Beach ceremony, which was also attended by UK Prime Minister David Cameron and German Chancellor Angela Merkel.
A Lancaster bomber, regarded as the workhorse of the RAF, flew over the crowds below, flanked by two Spitfires.