Tears and heartfelt tributes have marked the 75th anniversary of the D-Day landings.
Veterans, many of them visibly overwhelmed as they recalled the events on the beaches of northern France all those decades ago, watched and listened as the Queen was joined by Donald Trump and other world leaders in Portsmouth.
Fourteen countries joined UK to mark the commemorations to honour veterans of the largest amphibious military invasion in history.
The Queen told the crowds of veterans and dignitaries: "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, 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," the Queen continued.
"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."
Some 300 D-Day veterans, who are all now aged over 90, are also attending the event in Portsmouth - one of the key embarkation points for the offensive.
Theresa May read a letter from Captain Norman Skinner to his wife, Gladys, written two days before he set sail for Normandy.
In it, he talks of how he wished he could have told her of his feelings about his family, his wife, his two daughters, but that events had overtaken him.
Capt Skinner died on 'Sword' beach on 7 June. The letter and a photograph of his wife were found in his pockets.
Mr Trump read a prayer to the commemoration written by his predecessor Franklin D Roosevelt.
It was originally delivered to the US nation by President Roosevelt on the evening of D-Day.
President Trump read: "Almighty God: Our sons, pride of our nation, this day have set upon a mighty endeavour, a struggle to preserve our republic, our religion, and our civilisation, and to set free a suffering humanity."
Figures from every country that fought alongside the UK were present, as well as Prince Charles and members of the Armed Forces.
French President Emmanuel Macron and German Chancellor Angela Merkel attended, as well as prime ministers from Canada, Australia, New Zealand, Belgium, the Czech Republic, Greece, Luxembourg, the Netherlands, Norway, Poland, Slovakia and Denmark.
The Queen's arrival in the royal box was signalled by a fanfare from musicians from the Band of the Royal Marines and the Tri-Service orchestra performed the national anthem.
The orchestra performed John Williams' Hymn to the Fallen which opens the D-Day based war film Saving Private Ryan, and has become associated with Second World War remembrance and memorials.
The audience watched three veterans sharing their testimonies of D-Day in a pre-recorded video displayed on large screens.
Bert Edwards, a British veteran who served as an Able Seaman on HMS Bellona spoke of provided firing support to Omaha beach from the ship.
He said he was not nervous on the day just "apprehensive" adding: "At the time we had no idea it was as vicious as it was."
Canadian veteran Bob Roberts also spoke, he was a lance corporal in the North Shore (New Brunswick) Regiment and was among the first to land on Normandy's Juno beach on June 6.
American Eugene Deibler shared his memories of serving as a sergeant with the 501st Airborne Regiment and parachuting into Normandy at 1.30am on June 6, behind Utah beach.
As the event got under way a group of veterans came onto the event's large stage and were given a standing ovation.
French president Mr Macron read the last letter of a young resistance fighter Henri Fertet, executed at 16-years-old.
Before he began, he said in English: "First, let me thank you sincerely, on behalf of my nation."
Later in the day, on the other side of the Channel, some 280 paratroopers - joined by two surviving D-Day veterans - boarded a Dakota aircraft in Duxford, Cambridgeshire, to fly to Sannerville with the Red Devils, where they jumped and landed in fields used as a drop zone for the 8th (Midlands) ParachuteBattalion, who went on to destroy bridges in a bid to restrict German movements during the missions.
Harry Read, 95, was a signalman, part of the British Sixth Airborne Division, whose mission it was to jump in to German-occupied northern France, secure key bridges and destroy the Merville Gun Battery ahead of the amphibious landings.
He said: "There are not many of us left to do this kind of jump - we’re in short supply."
During the earlier commemorations, an extract from Second World War leader Winston Churchill's famous "We Shall Fight on the Beaches" speech was played.
The crowd heard: "We shall go on to the end, we shall fight in France, we shall fight on the seas and oceans, we shall fight with growing confidence and growing strength the air, we shall defend our Island, whatever the cost may be, we shall fight on the beaches, we shall fight on the landing grounds, we shall fight in the fields and in the streets, we shall fight in the hills; We shall never surrender."
D-Day was the start of Operation Overlord which saw the Allied forces launch a combined naval, air and land assault on Nazi-occupied France, and is considered a turning point in the Second World War.
However, it saw thousands killed and injured as it took place.
The Allied landings, early in the morning of June 6, 1944, on the Normandy beaches marked the start of a long and costly campaign to liberate north-west Europe from German occupation.
Airborne forces parachuted into drop zones across northern France, while ground troops then landed across five assault beaches - Utah, Omaha, Gold, Juno and Sword.
By the end of the day, the Allies had established a foothold along the coast and could begin their advance into France.
The commemoration event on Southsea Common in the Hampshire port city included an hour-long production telling the story of the invasion with testimony from veterans, theatrical performances and live music, as well as a flypast of the Red Arrows and Spitfires.
In her final official appearance as Prime Minister, Mrs May will also give a speech in which she will call for continued Western unity in tackling "new and evolving security threats".
"The Normandy landings 75 years ago were a moment of historic international co-operation," Mrs May will say.
"And it is right that at the heart of today's commemorations are the veterans who fought to secure the liberty and the peace that we now enjoy.
"The global challenges we face today are different in their origin and nature.
"But as we confront new and evolving threats to our security, it is more important than ever that we continue to stand together in upholding our shared values and way of life.
"That's why the UK has this week committed our Queen Elizabeth-class aircraft carriers and F-35 fighter jets to support the efforts of Nato forces to preserve the security and collective defence of our allies.
"As I host leaders from around the world today to mark this significant moment in our shared history, we will together reflect on the continued importance of the Western alliance for all our countries' security and prosperity.
"And as we unite to pay tribute to those whose bravery and sacrifice on the beaches of Normandy marked a turning point in the Second World War, we will vow never to forget the debt we owe them.
"Their solidarity and determination in the defence of our freedom remains a lesson to us all. And we will continue to stand up for the values of democracy, justice and tolerance that so many died to preserve."
The event will be the first time the UK has hosted this many world leaders outside a formal summit since the 2012 Olympics.
After the commemoration event, the world leaders will then have a reception with the veterans before sitting down to discuss the continued importance of the Western alliance and security.
From the deck of the aircraft carrier HMS Queen Elizabeth, Mrs May and Defence Secretary Penny Mordaunt will then wave off some 300 veterans as they retrace the journey they made across the Channel 75 years ago, followed by a flotilla of Royal Navy vessels.
More than 4,000 personnel will be involved in D-Day events in the UK and France, in what is set to be one of the biggest mobilisations of the UK armed forces in recent history.
On Thursday, Mrs May will commemorate the anniversary in Normandy at the inauguration of a memorial to British servicemen at Ver-sur-Mer, overlooking Gold Beach, as well as attending services of remembrance at the cathedral and cemetery in Bayeux.