Prince Charles, Theresa May, Donald Trump and other leaders are attending a remembrance ceremony in Bayeux to mark the 75th anniversary of D-Day.
The cathedral service is being held in Bayeux, the first major place to be liberated after the Allied forces invasion.
Following the service, veterans will parade from the cathedral to the Commonwealth War Graves cemetery.
Dignitaries will be invited to lay memorial wreaths and the last post will be played at the site where more than 4,000 war dead are buried as commemorative events are held in France and the UK.
Shortly after sunrise, a lone piper played an emotional lament at the exact moment British troops landed on Gold Beach in Normandy on June 6, 1944.
Pipe Major Trevor Macey-Lillie, of 19th Regiment Royal Artillery (The Scottish Gunners), performed Highland Laddie as crowds gathered on the beach below him and lined the promenade, applauding his performance.
The piper began the lament at 7.25am local time (6.26am BST) on the remaining Mulberry Harbour in the town called Port Winston, signalling the minute the invasion began and the moment the first British soldier landed on Gold beach in Nazi-occupied France.
D-Day was the largest amphibious invasion in military history and a major turning point for the Allies in the Second World War.
Before the cathedral service, Mrs May and French President Emmanuel Macron attended an inauguration ceremony for a memorial to commemorate more than 20,000 members of the British armed forces who died in Normandy in summer 1944.
The British Normandy Memorial, funded by the Normandy Memorial Trust, is being built on a hillside in Ver-sur-Mer, overlooking Gold Beach, one of the key sites for British troops during the Normandy Landings.
A sculpture created by David Williams-Ellis was unveiled marking the beginning of construction for the memorial depicting three British soldiers storming the beaches.
Expected to be completed within a year, it will record the names of 22,442 members of the British armed forces who died in the D-Day landings and Battle of Normandy.
Mrs May and Mr Macron laid wreaths at the foundation stone of the monument.
Seven British D-Day veterans were accompanied by four children, including Sir Winston Churchill's great-great grandson John Churchill, to lay flowers in front of the sculpture.
Addressing the audience, French President Emmanuel Macron said: "I am honoured to stand alongside Theresa May today to launch construction work for the British memorial at Ver-sur-Mer.
"The British people have long dreamt of this memorial."
He added: "This is where, 75 years ago, on June 6, 1944, almost 25,000 British soldiers landed in France to free the country from Nazi control.
"This is where young men, many of whom had never set foot on French soil, landed at dawn under German fire, risking their lives while fighting their way up the beach, which was littered with obstacles and mines."
He added: "It is time to remedy the fact that no memorial pays tribute to the United Kingdom's contribution to the Battle of Normandy."
He said the monument would also be a symbol of the ties binding France and the UK.
He said: "Nothing will break them. Nothing can ever break ties that have been bound in bloodshed and shared values.
"The debates taking place today cannot affect the strength of our joint history and our shared future."
President Macron assured Mrs May of his friendship, adding: "Leaders may come and go but their achievements remain.
"The force of our friendship will outlast current events."
Speaking at the inauguration service, Mrs Theresa May said: "It's incredibly moving to be here today, looking out over the beaches where one of the greatest battles for freedom this world has ever known took place.
"And it is truly humbling to do so with the men who were there that day.
"It's an honour for all of us to share this moment with you."
She added: "Standing here as the waves wash quietly onto the shore below us, it is almost impossible to grasp the raw courage it must have taken that day to leap from the landing craft and into the surf despite the fury of battle...
"If one day can be said to have determined the fate of generations to come in France, in Britain, in Europe and the world, that day was June 6, 1944."
Paying tribute to those who made the ultimate sacrifice, she said: "They laid down their lives so that we might have a better life and build a better world."
The ceremony concluded with a piped lament from Trooper Kurtis Rankin of The Royal Dragoon Guards, and Mrs May and Mr Macron then spoke to the veterans.
Some 300 plus veterans are in the town of Arromanches for a series of events.
Veterans will descend on the town square as part of a parade with Tobias Ellwood, the minister for Defence People and Veterans, and chief of Ministry of Defence general staff Mark Carleton-Smith will be among crowds before a Red Arrows flypast and a firework display.
In the UK, a service of remembrance and wreath laying takes place at the National Memorial Arboretum in Alrewas, Staffordshire.
In Portsmouth - a day after a national service attended by the Queen, Mrs May, US President Donald Trump and other world leaders - a veteran's parade will take place before a memorial service at the city's D-Day Stone.
And in London, Prince Harry will attend Founder's Day at the Royal Hospital Chelsea where he will see the Chelsea Pensioners and six veterans from the Normandy Landings.
Earlier, serving soldiers paid tribute to the heroic efforts of their Second World War brothers in arms at a ceremony marking the first key battle of D-Day.
Shortly after midnight on June 6, 1944, soldiers landed next to bridges over the river and canal at Benouville.
Capturing the two bridges, codenamed Ham and Jam, was a key part of the D-Day operation, as it hampered movement of enemy troops and enable the Allied forces to press forward from the beaches.
Dropping on French soil minutes into D-Day, parachutists and gliders from the Oxford and Buckinghamshire Light Infantry and the Glider Pilot Regiment joined the Royal Engineers, who were towed across the Channel in 28-men Horsa gliders, pulled by bombers, before gliding to their small landing area with pinpoint accuracy from 6,000 feet.
The bridges were captured after a battle lasting under 15 minutes in which two soldiers were killed and 14 wounded, including Herbert Denham 'Den' Brotheridge, who led the troops across and was the first British soldier killed in action in the Normandy campaign.
When mission commander Major John Howard transmitted the Ham and Jam code to signal their success it marked the first British objective to be achieved on D-Day.
Three-quarters of a century on, 130 soldiers from their successor units The Rifles and the Army Air Corps gathered at the bridge.
The Rifles held a ceremony at the Pegasus Bridge memorial to mark the moment the gliders landed, with a speech from Major Howard's daughter Penny.
French politician Christophe Blanchet told the crowd: "This bridge was to allow the liberation of France, and with it the rest of Europe."
A recording of Major Howard's speech from a ceremony at the site 30 years ago was played before the Band and Bugles of The Rifles led their troops across the bridge.
Rifleman Peter Ramsden, of the Rifles 2nd Battalion, spoke of his pride in following in the footsteps of D-Day heroes.
The 23-year-old from Durham, and currently stationed in Lisburn, Northern Ireland, said: "It's a massive honour to march across Pegasus Bridge.
"As one of our regiment's most prestigious battle honours, it means a lot not only to the regiment but to the lads in the regiment as well."
The officers and soldiers joined Arlette Gondree at for a Champagne toast at the cafe she owns next to the bridge, Cafe Gondree, in a tradition held since D-Day.
The cafe, where she lived with her family, was the first home in France to be liberated by the British and Ms Gondree's father dug up champagne he had hidden in the garden from the Nazi occupiers, to toast the British troops who freed them.
Ms Gondree said: "There were 54 Germans in the village, fortunately they never found daddy's hoard.
"But when the British soldiers dug out their trenches, my father asked them to dig it up.
"Thus the reason why the tradition of the Champagne is tied to the Gondree name, to this simple house, and this is what I do tonight to thank them all and to remember what they went through."
The great grandson of wartime prime minister Winston Churchill, Randolph Churchill, has paid tribute to those who took part in the D-Day landings.
"It's remarkable to think that 75 years ago the heroes that gave us our freedom were coming up these shores," he told ITV's Good Morning Britain.
"With the veterans here today - and it's wonderful to see so many - it's wonderful we can honour them and their comrades that didn't come back.
"They really did give their today so we can have a better tomorrow."
On Wednesday, as commemorative events were held on both sides of the Channel.
Dressed in red jumpsuits, 95-year-old Harry Read and 94-year-old John Hutton performed tandem jumps with the British Parachute Regiment's freefall display team, the Red Devils.
