The leaders of France and Britain paid tribute to the sacrifice of D-Day veterans on Thursday, the 75th anniversary of the largest ever seaborne invasion that opened the way for western Europe's liberation from Nazi Germany.

Inaugurating a memorial to the 22,000 soldiers under British command who were killed on June 6, 1944, and in the ensuing battle for Normandy, British Prime Minister Theresa May saluted the bravery of the soldiers, many of whom were still boys when they waded ashore under German fire.

"It's almost impossible to grasp the raw courage it must have taken that day to leap from landing craft and into the surf despite the fury of battle," May told a small gathering that included Macron and veterans, their uniforms laden with medals.

"These young men belonged to a very special generation ... whose incomparable spirit shaped our post-war world," she said. "They laid down their lives so that we might have a better life and build a better world."

The Normandy landings were months in the planning and kept secret from Nazi Germany despite a huge trans-Atlantic mobilisation of industry and manpower.

Under the cover of darkness, thousands of Allied paratroopers jumped behind Germany's coastal defences. Then, as day broke, warships pounded German positions before hundreds of landing craft disgorged the infantry troops under a barrage of machine-gun fire and artillery.

Some veterans say the sea turned red with blood during the operation that would help turn the tide of World Two against Hitler.

The devastation wrought by two world wars in the first half of the 20th century fostered a decades-long era of cooperation between European capitals determined to protect their hard-fought peace, giving rise to what is now the European Union.

But even as Britain now tries sever its ties with the bloc, Macron said some ties between France and Britain were indestructible.

"Nothing will ever take away the links of spilled blood and shared values. The debates of the present in no way take away from the past."

SACRIFICED LIVES

An hour after sunrise, under clear blue skies, a lone piper atop the remnants of an artificial harbour played Highland Laddie to mark the hour the first British soldier set foot on French sand. The Mulberry harbour was constructed to enable the resupply of allied troops as they pushed the Germans back.

Restored wartime jeeps and amphibious vehicles lined the beach at Arromanches and in villages along the Normandy shore the flags of Britain, Canada and the United States, the main contributors to the Allied force, fluttered in the breeze.

U.S. President Donald Trump and Canadian Prime Minister Justin Trudeau will also attend ceremonies along the stretch of coastline in northern France where more than 150,000 troops landed on five beaches - codenamed Gold, Juno, Sword, Utah and Omaha by the Allies.

Lines of white marble crosses on the clifftop above Omaha Beach today mark the resting place of more than 9,380 U.S. soldiers who died during the military campaign.

Here, Trump will watch Macron award the Legion d'Honneur, France's highest award for merit, to five U.S. veterans before the two presidents leave for a working lunch in nearby Caen.

"UNBREAKABLE BOND"

Trump will pay tribute to the U.S. soldiers who sacrificed their lives "for the survival of liberty," according to the White House.

"To all of our friends and partners: our cherished alliance was forged in the heat of battle, tested in the trials of war, and proven in the blessings of peace. Our bond is unbreakable," the U.S. president is expected to say.

The commemorations come against the backdrop of two years of forthright diplomacy and "America First" policymaking by Trump and his administration that have shaken the NATO alliance and tested relations with erstwhile allies including Britain and France.

On the eve of the anniversary, France's president evoked the spirit of D-Day, saying: "These allied forces that together freed us from the German yoke, and from tyranny, are the same ones that were able to build the existing multilateral structures after World War Two."

"We must not repeat history, and remind ourselves what was built on the basis of the war."

Commemoration events began in France on Wednesday. Khaki parachutes filled the skies above Sannerville as two British veterans in the mid-nineties joined several hundred paratroopers re-enacting the jumps behind enemy lines. Hundreds of British veterans crossed the English Channel on a specially chartered ferry.