With guests waving UK flags, appearances from veterans, recollections and readings - and some tears in Queen Camilla's eyes - the British ceremony took place in Portsmouth, the main departure point for the 5000 ships that headed to Normandy for the June 6, 1944, operation.
"Today we come together to honour those nearly 160,000 British, Commonwealth and Allied troops who, on 5th June 1944, assembled here and along these shores to embark on the mission which would strike that blow for freedom and be recorded as the greatest amphibious operation in history," King Charles said.
"So, as we give thanks for all those who gave so much to win the victory, whose fruits we still enjoy to this day, let us, once again, commit ourselves always to remember, cherish and honour those who served that day and to live up to the freedom they died for."
About 4400 Allied troops died on D-Day.
"We will always remember those who served and those who waved them off," Prince William said.Â
"The mothers and fathers, brothers and sisters, sons and daughters who watched their loved ones go into battle, unsure if they would ever return."
An emotional letter written to his wife by a soldier killed the day after D-Day was read out.
Prime Minister Rishi Sunak read out the message from General Bernard Montgomery, commander of the Allied forces on D-Day, which was delivered to all troops ahead of the invasion.
Meanwhile in France, where the main ceremonies will take place on Thursday with world leaders including US President Joe Biden and Ukrainian President Volodymyr Zelenskiy, thousands of tourists could be seen alongside the D-Day beaches and visiting World War II cemeteries.
Collectors drove army jeeps and US, Canadian, UK and French flags adorned buildings.
"It's very important not to forget this sacrifice," British tourist Daniel Reeves, 27, said as he visited the US war cemetery in Colleville-sur-Mer earlier this week.
It's "absolutely amazing and extremely emotional, especially when you see them, the veterans, and they say: Thank you my friend," British visitor Karen Swinger said.
French President Emmanuel Macron, who paid tribute to French resistance fighters in Brittany earlier in the day, will later take part in a ceremony in Saint-Lo, Normandy, which was almost entirely destroyed by Allied bombardments as part of D-Day and the Battle of Normandy.
"This trauma turned our city into the 'capital of the ruins', as playwright Samuel Beckett wrote," the city's mayor Emmanuelle Lejeune told Reuters.
The city of 12,000 people at the time was 90 per cent destroyed.
