French President Francois Hollande has led the D-Day 70th anniversary tributes by remembering those who died on a day that "changed the world".
He attended a service near Bayeux with US President Barack Obama, who said the US commitment to liberty was "written in blood" on French beaches. The Queen laid a wreath at a ceremony nearby.
They will gather at Sword Beach, one of five landing points for Allied troops.
The landings 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.
At the D-Day commemorations
Caroline Wyatt, BBC defence correspondent
As the sun rose over Normandy shores this morning, a veteran watched, lost in memories, from the deck of HMS Bulwark. The Royal Navy flagship had sailed the English Channel overnight at the head of an international task group of ships.
For former Royal Marine Corporal Bill Bryant, 89, the sight of the beaches brought back emotional recollections of the same time exactly 70 years ago, as he prepared to drive his landing craft to the shores - carrying his colleagues to their fate on land, amid a barrage of noise and chaos.
The contrast with today could not have been greater as he joined many other veterans on "Gold" Beach, amid a festival atmosphere. The sunshine sparkled on the waves, and French families and tourists from across Europe gathered to watch military bands on the main square at Arromanches.
This doughty but dwindling band of brothers know this may be the last time they meet on these shores. For the veterans, and those who've come to honour them, the ceremonies at Bayeux cemetery are a poignant but powerful reminder of courage and endurance, as D-Day slowly passes from living memory into history.
During his speech at a US war cemetery at Colleville-sur-Mer near Bayeux, President Hollande said every man who took part in D-Day was a "hero".
He said France would never forget what it owed them, nor would it forget its solidarity with the US.
Also at Colleville-sur-Mer, President 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".
Speaking earlier in the city of Caen, President Hollande said: "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."
In nearby Bayeux, the Queen laid a wreath at a remembrance service at Commonwealth War Graves Cemetery, where she, the Duke of Edinburgh, Prince Charles and the Duchess of Cornwall also met veterans.
Prince Charles and the Duchess of Cornwall attended a Royal British Legion service at Bayeux Cathedral.
The Duke and Duchess of Cambridge will attend events in Arromanches, near Gold landing beach where thousands of British troops came ashore on D-Day.
In other events:
- A flotilla of ships from the Allied nations led by HMS Bulwark sailed from Portsmouth to Normandy on Thursday
- Prince Charles and The Duchess of Cornwall will attend a remembrance ceremony at the Juno Beach Centre, Courseulles-sur-Mer, to commemorate Canada's role in the Normandy landings
- The Duke and Duchess of Cambridge are to attend a veterans' tea party in Arromanches
The day's commemorations began at midnight with a vigil at Pegasus Bridge near Ouistreham, marking the first assault of the D-Day invasion when British soldiers began the first Allied action of the campaign.
At 00:16 on 6 June 1944, six Horsa gliders carrying 181 men from the Glider Pilot Regiment and the 2nd Battalion, Oxfordshire and Buckinghamshire Light Infantry, landed silently to capture the strategically-vital bridge and another nearby.
Heads of state, including the Queen, Mr Hollande, US President Barack Obama and Russia's Vladimir Putin will meet at Ouistreham - the beach codenamed Sword. Around 650 UK veterans are also expected to gather there.
At the Royal British Legion service, national chaplain, the Right Reverend Nigel McCulloch, said: "What you achieved here in Normandy and beyond validated the sacrifices made earlier in the war by thousands who, like my father, were killed in action".
He added that the "self-concern and love for others" shown by those involved in D-Day "is the route of human justice, freedom and peace".
In a reading at the cathedral, Brigadier David Baines MBE spoke of men who died "with no memorial".
What was D-Day?
On 6 June 1944, British, US and Canadian forces invaded the coast of northern France in Normandy.
The landings were the first stage of Operation Overlord - the invasion of Nazi-occupied Europe - and were intended to end World War Two.
Portsmouth's D-Day Museum says as many as 4,413 Allied troops died on the day of the invasion - more than previously thought.
By the end of D-Day, the Allies had established a foothold in France. Within 11 months Nazi Germany was defeated, as Soviet armies swept in from the east and captured Hitler's stronghold in Berlin.
He added: "But these were merciful men whose righteousness has not been forgotten."
Cpl Bill Bryant, of the Royal Marines, has returned to Normandy to mark the anniversary and he told the BBC that "all hell was being let loose" as he landed on the beach 70 years ago.
"I was scared out of my life," he said, adding that it was emotional to return to northern France as it brought back "too many memories".
A 21-gun salute and a flypast will form part of proceedings at Friday's ceremony, which will be attended by UK Prime Minister David Cameron and German Chancellor Angela Merkel.
Mr Putin will be at the ceremony with Prince Charles, who reportedly criticised the Russian president while speaking to a woman during a tour in Canada.
Ukraine's President-elect Petro Poroshenko is expected to attend, amid tensions between his country and Russia.
In face-to-face talks in Paris, Mr Cameron said he gave Mr Putin a "very clear and firm set of messages" about the crisis in Ukraine.