French President Emmanuel Macron thanked Allied Forces on behalf of his country as he joined the Queen and hundreds of veterans at a national commemorative ceremony to honour those who fought in the D-Day landings.
Mr Macron is among world leaders representing the Allied nations involved in Operation Overlord, including Canadian Prime Minister Justin Trudeau and US President Donald Trump, who is coming to the end of a three-day state visit to the UK.
Other guests at the Portsmouth event marking 75 years since D-Day include 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.
Three hundred veterans are also present, while 60,000 members of the public are expected to attend the Portsmouth Naval Museum.
A segment of the event saw a tribute to the agents of the Special Operations Executive, a unit which supported the French Resistance.
Mr Macron read the last letter of a young resistance fighter Henri Fertet, executed at just 16-years-old.
Before he began, he said: “First, let me thank you sincerely, on behalf of my nation.”
Also joining the ceremony are 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 then posed for a group photograph with the monarch and the Prince of Wales.
President Trump read a prayer written by his predecessor Franklin D Roosevelt.
He read the piece which was originally delivered to the US nation by President Roosevelt on the evening of D-Day – June 6 1944.
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.
A guard of honour formed of military personnel from the Royal Navy, Army and RAF marched through the spectator aisles and on to the main stage.
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.
Speaking ahead of the service, Sergeant John Jenkins, 99, from Portsmouth, said he was “overwhelmed” to be a part of the day.
“Obviously, I will think of all my mates that didn’t come back,” he said.
“I can’t say any particular one because we were all comrades together, that was the thing.
“We were all comrades together and that’s what carries us through – the comradeship.”
He was serving with the Pioneer Corps on D-Day and landed on Gold Beach on June 8 1944.
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.
Mrs May will be making her final official appearances as the British Prime Minister during the D-Day commemorations, which continue on Thursday across Normandy.
She will call for unity in western Europe, saying: “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.
“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.”
An hour-long production telling the story of the invasion was being played to the crowds, featuring testimony from veterans, before theatrical performances, live music and a flypast.
Afterwards, world leaders were meeting to discuss the western alliance and security following a reception with veterans.
From the deck of the aircraft carrier HMS Queen Elizabeth, Mrs May and Defence Secretary Penny Mordaunt will then wave off the crowds of veterans who are set to retrace the journey they made across the channel 75 years ago.
This time they will be followed by a flotilla of Royal Navy vessels.
Meanwhile, hundreds of veterans are flocking to northern France and Portsmouth as well as to events around the country to mark the occasion.
On Wednesday afternoon, 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.