The Queen thanked veterans and hailed the resilience of “the wartime generation” as world leaders honoured those who fought in the D-Day landings.
More than 300 veterans joined leaders representing the Allied nations involved in Operation Overlord at the Portsmouth event marking 75 years since D-Day.
Prime Minister Theresa May, French President Emmanuel Macron, Canadian Prime Minister Justin Trudeau and US President Donald Trump were among those at the national commemorative ceremony.
Thousands of members of the public attended the Portsmouth Naval Museum to watch the hour-long service.
Before reading the letter of a young resistance fighter, President Macron said: “First, let me thank you sincerely, on behalf of my nation.”
The Queen stood in the royal box next to her son, the Prince of Wales, as she paid tribute to the resilience of her generation.
“When I attended the commemoration of the 60th anniversary of the D-Day landings, some thought it might be the last such event,” she said.
“But the wartime generation – my generation – is resilient, and I am delighted to be with you in Portsmouth today.
“Seventy-five years ago, hundreds of thousands of young soldiers, sailors and airmen left these shores in the cause of freedom.
“In a broadcast to the nation at that time, my father, King George VI, said: ‘… what is demanded from us all is something more than courage and endurance; we need a revival of spirit, a new unconquerable resolve …’
“That is exactly what those brave men brought to the battle, as the fate of the world depended on their success.
“Many of them would never return and the heroism, courage and sacrifice of those who lost their lives will never be forgotten.
“It is with humility and pleasure, on behalf of the entire country – indeed the whole free world – that I say to you all, thank you.”
Musicians from the Band of the Royal Marines played a fanfare when the Queen arrived in the royal box, with the Tri-Service Orchestra performing the national anthem.
Guests at the event included German Chancellor Angela Merkel, Australian Prime Minister Scott Morrison, premier Charles Michel from Belgium, Czech Prime Minister Andrej Babis and President Prokopis Pavlopoulos from Greece.
Also joining the ceremony were the prime ministers of Luxembourg, Xavier Bettel; the Netherlands, Mark Rutte; Norway, Erna Solberg; and Poland, Mateusz Morawiecki, and Slovakia’s deputy prime minister Richard Rasi.
They all met the Queen before the event began – a first for Mr Macron – and posed for a group photograph with the monarch and the Prince of Wales.
During the event, Mr Macron read a letter in French of Henri Fertet, who was executed aged 16.
It included the line: “I am going to die for my country. I want France to be free and the French to be happy.”
President Trump read a prayer written by his predecessor Franklin D Roosevelt and delivered to the to the nation on the evening of D-Day.
When the president left the stage, there was a performance of the Boogie Woogie Bugle Boy of Company B, a wartime classic by the Andrews Sisters.
Mrs May read a letter from Captain Norman Skinner of the Royal Army Service Corps, to his wife Gladys on June 3 1944.
The letter was in his pocket when he landed on Normandy’s Sword Beach on D-Day but he was killed the following day, leaving his wife and two young daughters.
Part of it read: “Although I would give anything to be back with you, I have not yet had any wish at all to back down from the job we have to do.”
Sergeant John Jenkins, 99, from Portsmouth, told those at the service of his experiences on D-Day.
He was just 23 when he landed on Gold Beach on June 8 and “terrified”, he said.
“I was terrified – I think everyone was,” he told the event.
“I look back on it as a big part of my life. I was just a small part in a very big machine.
“It is an honour to be stood here.
“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.”
After the event, the Queen, Mr Trump, the First Lady and Prince Charles met with six veterans.
Thomas Cuthbert, 93, who was in a landing barge oiler anchored off shore from Utah and Omaha beaches, then worked along the coast off Gold, Juno and Sword beaches.
He said of the president: “He came across very well, he surprised me, he seemed one of the boys.”
Considered a turning point in the Second World War, Operation Overlord saw thousands killed and injured after its launch on June 6 in 1944.
The commemoration event marks the first time the UK has hosted this many world leaders outside a formal summit since the 2012 Olympics.
After a reception with veterans, world leaders met to discuss the western alliance and security.
On Wednesday evening, from the deck of aircraft carrier HMS Queen Elizabeth, Mrs May and Defence Secretary Penny Mordaunt will wave off crowds of veterans who will retrace the journey they made 75 years ago.
Around 250 veterans are making the voyage on the MV Boudicca, a cruise ship chartered by the Royal British Legion to mark the anniversary.
They will be followed by a flotilla of Royal Navy vessels.
Two veterans, Harry Read, 95, and John Hutton, 94, will parachute into Normandy in honour of comrades they lost when they first made the descent 75 years ago.
Alongside around 280 paratroopers, they will take part in the jump onto fields at Sannerville – the drop zone for the 8th Midlands Parachute Battalion during D-Day.
In the evening, a vigil and silent march will take place at Pegasus Bridge which was the scene of a 15-minute skirmish to take hold of the pathways over the Caen Canal and River Orne.
This was one of the first places British troops liberated on D-Day.