King Charles paid a heartfelt tribute to those who had taken part in the D-Day landings, praising them for “replacing tyranny with freedom”.
“We are eternally in their debt,” the King told an 80th anniversary commemoration.
He was speaking in Portsmouth, one of the key departure points for the Normandy landings in June 1944.
The King hailed the “courage, resilience and solidarity” of those who had taken part in D-Day and whose numbers were now “dwindling to so few”.
King Charles, with Queen Camilla and the Prince of Wales, was addressing a national D-Day commemoration held on Southsea Common.
He hailed the “greatest amphibious operation in history” and the courage of those who “must have questioned if they would survive”.
The King said their efforts to end “brutal totalitarianism” must never be forgotten.
And he called on the present generation to honour those who had died in ways that “live up to the freedom they died for, by balancing rights with civic responsibilities”.
Portsmouth was one of the embarkation points on the south coast 80 years ago, as Allied forces crossed the Channel to liberate France and Western Europe from Nazi occupation.
The commemorative event heard from those who took part in D-Day, including Roy Hayward, who had landed in Normandy on 6 June at the age of 19.
Mr Hayward said he wanted to remember those who had “fought for democracy” and “to ensure their story is never forgotten”.
Prince William delivered a poignant reading from the diary of Captain Alastair Bannerman, in which the soldier remembered his family as he headed towards the French coast on the morning of D-Day.
This year, 23 surviving D-Day veterans are travelling for commemorations in Normandy and a further 21 will be at a memorial event at the National Memorial Arboretum in Staffordshire.
There were 225 D-Day veterans able to travel to Normandy five years ago, and the Royal British Legion has said these “poignant commemorations will be our last opportunity to host a significant number of Normandy veterans”.
Last week the King met one of the veterans of the Normandy landings – Jim Miller – who at the age of 20 had gone ashore at Juno Beach.
The King invited Mr Miller to Buckingham Palace to personally hand him his 100th birthday card.
“I am humbled to reach such a great number, especially when I think of those who fell on the Normandy beaches all those years ago,” Mr Miller said afterwards.
The King will travel to France for a commemorative event on Thursday at the British Normandy Memorial at Ver-sur-Mer.
An international ceremony with more than 25 heads of state will be attended by Prince William.