The Queen has paid tribute to the "heroism, courage and sacrifice" of those who died in the D-Day landings.
She was joined by 16 world leaders, including US President Donald Trump, in Portsmouth to commemorate the 75th anniversary of history's largest combined land, air and naval operation.
Veterans of the landings in Normandy to liberate western Europe also attended.
Quoting a broadcast by her father, King George VI, at the time of the operation in World War II, the Queen said the veterans of D-Day demonstrated "more than courage and endurance", showing "unconquerable resolve".
"The fate of the world depended on their success," she said. "Many of them would never return, and the heroism, courage and sacrifice of those who lost their lives will never be forgotten."
She thanked them "with humility and pleasure, on behalf of the entire country, indeed the whole free world".
The countries represented at the event have agreed to make a joint statement pledging to ensure the "unimaginable horror" of the war is not repeated.
Called "the D-Day proclamation", the 16 signatories - including the UK and the US - will commit to working together to "resolve international tensions peacefully".
On Thursday, further memorial services are planned to mark the 75 years since the D-Day landings on 6 June 1944 - the start of the campaign to liberate Nazi-occupied north-west Europe.
The Queen told the crowd she was "delighted" to be able to thank veterans for their service.
She added: "When I attended the commemoration of the 60th anniversary of the D-Day landings, some thought it might be the last such event.
"But the wartime generation, my generation, is resilient."
Members of the armed forces and more than 300 veterans, who are all over 90 years old, attended the event in Portsmouth - one of the key embarkation points on D-Day.
Sgt John Jenkins, who is 99 and served with the Pioneer Corps in the Normandy landings, said: "I was terrified, I think everyone was.
"You never forget your comrades because we were all in it together. It is right that the courage and sacrifice of so many is being honoured 75 years on. We must never forget."
Hundreds of other veterans are in northern France to mark the occasion there.
D-Day, officially known as Operation Overlord, began when the Allies launched an aerial bombardment on the German lines in Normandy before dropping 18,000 troops there on the night of June 5, 1944, to prepare the way for the seaborne invasion of occupied France.
The following day Operation Neptune succeeded in landing 132,000 ground troops on the beaches.
Leaders from every country that fought alongside the UK on D-Day joined the Queen and the Prince of Wales for the commemorations on Southsea Common.
They included French President Emmanuel Macron, US President Donald Trump and Canadian Prime Minister Justin Trudeau.
Also attending were German Chancellor Angela Merkel, as well as leaders from Australia, Belgium, Czech Republic, Denmark, Greece, Luxembourg, the Netherlands, Norway, New Zealand, Poland and Slovakia.
The commemoration featured an hour-long production telling the story of the invasion, with testimony from veterans, theatrical performances and music. Veterans saluted the crowd, and actress Sheridan Smith performed a Dame Vera Lynn song.
Thousands of members of the public, separated from the VIPs and veterans by a large security fence, watched the events live on big screens on Southsea Common.
A designated protest area was set up in Guildhall Square, 1.6km from the Southsea Common events. Civic leaders were worried any protests near the main event might upset the veterans.
Later in the afternoon, veterans Harry Read, 95, and John Hutton, 94, were due to honour their lost comrades by parachuting into Normandy.
At Caen, France, a few hours after attending the ceremony in the UK, President Macron paid tribute to 71 French Resistance fighters shot dead by the Gestapo on the day Allied forces landed in Normandy but whose bodies were never recovered.
- BBC / Reuters