Mr Brown, Mr Sarkozy, Mr Obama and Canadian Prime Minister Stephen Harper each spoke at a ceremony at the Normandy American Cemetery, where more than 9,000 men are interred.
It is close to Colleville-Sur-Mer, which was codenamed Omaha Beach in 1944, one of five beaches targeted at the start of the allied invasion on German-occupied France.
Mr Brown said: "Sixty five years ago in the thin light of grey dawn more than 1,000 small craft took to a rough sea on a day that will be forever a day of bravery.
"On that June morning the young of our nations stepped out on those beaches below and into history. As long as freedom lives their deeds will never die."
British veteran Jack Woods, 85, was presented with the rank of Officer of the Legion d'Honneur, France's highest military award, by President Sarkozy.
Nine other Britons had already been made Knights of the Legion d'Honneur, to mark the anniversary.
Mr Obama praised the British Blitz spirit in the "story of the Allied victory".
By the BBC's Robert Hall, in Normandy
After a week of clear skies, we and they had hoped for better...
Now their blazers and crisp white shirts were sodden, water dripping from the medals on their chest and, yet, few abandoned their post.
These men had come to honour the fallen and to mark a day which changed so many lives; they were not going to give up that moment.
This event, in Arromanches town square, always has an informal feel; perhaps too informal said some veterans, frustrated by delays to the ceremony.
But when those still fit enough to march swung into the square with a skirl of pipes, the magic returned.
The presence of Gordon Brown and his French counterpart was almost incidental; old friends could once more stand behind the standards, and keep silence for the friends who still lie here in France.
"It is the legend of units like Easy Company and the All-American 82nd," he said.
"It is the tale of the British people, whose courage during the Blitz forced Hitler to call off the invasion of England. The Canadians, who came even though they were never attacked.
"The Russians, who sustained some of the war's heaviest casualties on the Eastern front and all those French men and women who would rather have died resisting tyranny than lived within its grasp."
The main British event of the day was a memorial service in Arromanches where thousands of UK and Canadian troops came ashore on 6 June 1944 and during the following days.
Mr Brown - facing turmoil over his leadership at home - was booed by a small group in the waiting crowd, and one person shouted: "Where's the Queen?"
The French government previously denied it had intended to snub the Royal Family after failing to invite the Queen to the commemorations, with Prince Charles travelling to France after a last-minute invitation.
Mr Brown gave a speech in which he praised "men of courage and bravery".
'Never, ever forget'
UK Chief of the General Staff, Gen Sir Richard Dannatt, told the veterans: "I know that I speak on behalf of all soldiers, all over the world, when I salute the focus, determination and professionalism of all of you and your comrades who stormed into northern France from the air and from the sea in June 1944."
Earlier, Mr Brown and the prince laid wreaths inside Bayeux cathedral during a service organised by the Royal British Legion in remembrance of the soldiers who lost their lives during the landings.
Some veterans have returned to the beaches for the first time, while others who have since died have been represented by relatives.
UK veterans returned to Sword beach
Veteran Frederick Wright told the BBC: "For years I wouldn't come but now I'm here I'm really enjoying it. The people appreciate us so much."
With his great-grandson, Tom Bagley, by his side, he added: "I hope the children, as they grow up, will never ever forget it."
Jackie Hopkins returned to the beach at Arromanches where her great-uncle Jack had landed aged just 21.
Clutching a Bible with a bullet-hole in it, she said her uncle had been fired at as he came up the beach, and a bullet penetrated the rucksack he was carrying in front of him, and lodged itself in his Bible.
"I wanted to come back here today in honour of his memory," she said.
Parades, marches and services have been taking place at dozens of towns and villages along the 50-mile stretch of coast where more than 4,000 members of the UK, US and Canadian forces lost their lives on the first day alone.
Chief of Defence Staff Sir Jock Stirrup, Defence Secretary Bob Ainsworth and French Prime Minister Francois Fillon were also present at the service in Bayeux, alongside Normandy veterans, serving servicemen and women and sea, Army and air cadets.
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