Joining May and Trump, who is on a three day state-visit to the UK, were French President Emmanuel Macron and German Chancellor Angela Merkel, as well as Prime Ministers from Australia, Belgium, Canada, Czech Republic, Denmark, Greece, Luxembourg, the Netherlands, Norway, New Zealand, Poland and Slovakia.
May who is making her final official appearances as Prime Minister ahead of her formal resignation on Friday described the landings as a "moment of historic international cooperation".
Events in Portsmouth retold the story of the build-up to the Battle of Normandy in France, which will host commemorations to be attended by world leaders on Thursday marking the June 6, 1944 day of the landings.
It remains the largest amphibious assault in history, claiming the lives of an estimated 4,400 troops in one day.
"Well, I was never a fan of that war I'll be honest with you. I thought it was a terrible war, I thought it was very far away," Trump said in reference to the war, during an interview with Britain's ITV channel ahead of the event.
"That may have been the greatest battle ever in history. Being at that site (Portsmouth) will be very interesting," he noted.
The US President covered a range of subjects during the interview with TV host Piers Morgan, known as close to Trump.
He revealed he had carried out a "great conversation" about climate change with Britain's heir to the throne Prince Charles, who is known for his strong views on global warming and quite obviously differs from Trump's stance of climate change denial.
The US President claimed he was "moved" by Charles; passion on the matter but asked whether he personally believes in climate change, Trump said "there is a change in weather and I think it changes both ways".
He also distanced himself from some reports that he had branded Meghan Markle, now Duchess of Sussex and the wife of Prince Harry, as "nasty" and claimed his reference was to the "nasty" comments the former actress had made about him.
Following the commemorations in Portsmouth, Trump will fly to Shannon for his first visit to the Republic of Ireland as US President. He will then hold a meeting with the Indian-origin Irish Prime Minister Leo Varadkar before he goes to his golf resort in Doonbeg.
It will mark the end of his state visit, which began with a welcome at Buckingham Palace and a series of grand events and state banquet. Addressing a joint press conference with Theresa May on Tuesday, Trump said that he wanted a "very, very substantial" trade deal with the UK once it had left the European Union (EU).
He also waded into the ongoing Conservative Party leadership contest for the successor to Theresa May, having held a phone conversation with frontrunner Boris Johnson and a one-to-one chat with fellow contender UK foreign secretary Jeremy Hunt and also reportedly a conversation with UK environment secretary Michael Gove.
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