The veterans' hands, which once wrested France from Nazi occupation, saluted wizened faces. Some rose to their feet with difficulty. Thousands of onlookers applauded.
"Thank you for having been here in the summer of '44. Thank you for still being here on June 6, 2014," Hollande told veterans and dignitaries on Sword Beach, one of five code-named beaches taken by the Allies 70 years ago. France's gratitude, he said, "will never be extinguished."
Earlier, he paid special tribute to U.S. soldiers buried at the Normandy American Cemetery. "Vive l'Amerique! Vive la France! And long live the memory of those who fell here for our liberty."
Taking the stand on Omaha Beach, a site he called "democracy's beachhead," Obama said: "America's claim — our commitment to liberty, to equality, to freedom, to the inherent dignity of every human being — that claim is written in blood on these beaches, and it will endure for eternity."
In all, 19 world leaders, more than 1,000 veterans and many others gathered to honor the troops and civilians who fell in mighty battles that helped bring Europe peace and unity.
At 6:30 a.m., the moment on June 6, 1944, when Allied troops first waded ashore, a U.S. military band played taps. D-Day veterans from the 29th Infantry Division and serving soldiers stood at attention.
"Twenty-nine, let's go!" they shouted, then downed shots of Calvados, Normandy's apple brandy.
Hundreds of Normandy residents and other onlookers applauded, then formed a human chain on the beach.
The glorious sun that rose as they arrived shone through the day on a land where paratroopers' corpses once hung from trees and medics dragged wounded soldiers from blood-swirled waves.
But the peace and stability that its wartime history brought continues to be challenged, as bloodshed in Ukraine poses new threats to European security and East-West relations.
Hollande sought to use Friday's gathering to reconcile Russia with the West and Ukraine, and invited Ukraine's president-elect as well as Russian President Vladimir Putin. Putin met with Petro Poroshenko and Obama on the sidelines of the event.
"It is because France itself experienced the barbarity (of war) that it feels a duty to preserve peace everywhere, at the frontiers of Europe as in Africa," Hollande said.
He was hosting the world leaders at a chateau in Benouville used as a hospital during the war, and in Ouistreham, a small port that was the site of a strategic battle on D-Day.
The leaders and veterans watched an elaborate interpretive dance on the beach that traced the horrors of World War II and Europe's painful road to healing afterward. Wartime film clips and evocative music accompanied the performance, which ended with fighter planes releasing red, white and blue smoke and gas flares shooting multicolored flames above the waves.
The secretly planned Operation Overlord included landings on five Normandy beaches, code-named Gold, Juno, Omaha, Sword and Utah.
The D-Day invasion was a turning point in World War II, cracking Hitler's western front as Soviet troops made advances in the east. At least 4,400 Allied troops were killed the first day, and many thousands more in the ensuing three-month Battle of Normandy, before the Allies could march to Paris to liberate the French capital from Nazi occupation.