At the British ceremony in Ver-sur-Mer, veterans were applauded as they filed into the event to take their seats, which were decorated with bright red poppies.
"I want to pay my respects to those who didn't make it. May they rest in peace," veteran Joe Mines said, in words read by actor Martin Freeman. "I was 19 when I landed, but I was still a boy... And I didn't have any idea of war of war and killing."
"I tried to forget D-Day but I can't," Royal Navy veteran Ron Hendrey said, in words read by actor Douglas Booth. "I've lived 80 years since that day, my friends have remained under the earth."
Both veterans were present at the ceremony.
With war raging in Ukraine, on Europe's borders, this year's commemoration of this major turning point in World War Two carries special resonance.
The anniversary takes place in a year of many elections, including for the European Parliament this week and in the U.S. in November. Leaders are set to draw parallels with World War Two and warn of the dangers of isolationism and the far-right.
Solemn music was played and Tom Jones sang "I won't crumble with you if you fall," in a ceremony attended by King Charles and Queen Camilla, and French President Emmanuel Macron and his wife Brigitte.
Charles, in full military dress, was visibly moved as he paid tribute to those who took part in the landings, as well as the French resistance.
"We recall the lesson that comes to us again and again across the decades: free nations must stand together to oppose tyranny," he said.
"Our admiration is eternal," said Charles, who spoke in both French and English. "Let us pray such sacrifice will never be made again."
Earlier in the day, as the sun rose in Arromanches-les-Bains, one of the beaches where Allied troops came ashore 80 years ago, small crowds filtered onto the beach to watch a collection of Second World War jeeps and an amphibious vehicle coming ashore carrying a bagpiper blasting a somber tune.
With the numbers of veterans, many aged 100 or more, fast dwindling, this is likely to be the last major ceremony in Normandy honouring them in their presence.
'TERRIBLE'
Some 200 veterans, most of them American or British, are set to take part in ceremonies throughout the day on windswept beaches that still bear the scars of the fighting that erupted on D-Day, history's largest amphibious invasion, in which thousands of Allied soldiers died.
At Omaha beach, the largest of the D-Day landing areas, where about 2,400 U.S. servicemen lost their lives on June 6, more than 20 heads of state and government were due to attend an international ceremony later in the day.
Landing craft were in place to reenact part of the landings, while several warships and patrol boats anchored on the horizon.
Among those who will take part in the ceremonies is 101-year old Bob Gibson, who was in the second wave of soldiers to land on Normandy's Utah beach.
"It's like it happened yesterday. You wouldn't believe what I have seen. Terrible. Some of the young fellows never reached the major beach ... sometimes it wakes you up at night," he told Reuters.
Ukrainian President Volodymyr Zelenskiy, German Chancellor Olaf Scholz and many others will take part in the day of tributes.
But Russia, which launched a full-scale invasion of Ukraine in 2022, touching off Europe's biggest armed conflict since World War Two, was not invited.
"Ukraine needs the support from the Europeans and the Allies like France and the other European states needed it to defeat Nazism," a European diplomat said, stressing it was important Zelenskiy will be there.
(Reporting by Elizabeth Pineau, John Irish, Lucien Libert, Jeff Mason in Normandy, Muvija M and William James in London; Writing by Ingrid Melander; Editing by Alexandra Hudson and Toby Chopra)

France's President Emmanuel Macron (C) reacts after addressing British D-Day veterans, including 104-years-old British World War II veteran, Christian Lamb (2-R), who helped to plan the D-Day landings in Normandy, as Britain's Queen Camilla (L) and Britain's King Charles III (2-L) look on during the UK Ministry of Defence and the Royal British Legions commemorative ceremony marking the 80th anniversary of the World War II 'D-Day' Allied landings in Normandy, at the World War II British Normandy Memorial near the village of Ver-sur-Mer which overlooks Gold Beach in northwestern France, 06 June 2024. The D-Day ceremonies on 06 June this year mark the 80th anniversary since the launch of 'Operation Overlord', a vast military operation by Allied forces in Normandy, which turned the tide of World War II, eventually leading to the liberation of occupied France and the end of the war against Nazi Germany. EPA-EFE/LUDOVIC MARIN / POOL MAXPPP OUT