“It was all very secret,” he added.
Dame Helen Mirren praised the bravery of the veterans in attendance during her introduction to the event at 11:00 BST, while Prime Minister Rishi Sunak read an address to the crowds.
Portsmouth was one of the embarkation points on the south coast eight decades ago, as Allied forces crossed the Channel to liberate France and Western Europe from Nazi occupation.
Foundations for the Allied victory were laid by the success of the Normandy landings, in which troops from the UK, US, Canada, and France conducted the largest seaborne invasion in history.
The commemorative event heard from those who took part in D-Day, including Roy Hayward, who landed in Normandy on 6 June 1944 at the age of 19.
Mr. Hayward, now aged 98, said he wanted to remember those who had “fought for democracy” and “to ensure their story is never forgotten”.
Last week the King met one of the veterans of the Normandy landings, Jim Miller, who at the age of 20 had gone ashore at Juno Beach.
The King invited Mr Miller to Buckingham Palace to personally hand him his 100th birthday card.
“I am humbled to reach such a great number, especially when I think of those who fell on the Normandy beaches all those years ago,” Mr. Miller said afterward.
Earlier on Wednesday, 21 veterans attended a memorial event at the National Memorial Arboretum in Staffordshire.
A further 23 surviving D-Day veterans attended commemorations in Normandy, where they were joined by Princess Anne.
There were 225 D-Day veterans able to travel to Normandy five years ago, and the Royal British Legion has said these “poignant commemorations will be our last opportunity to host a significant number of Normandy veterans”.
On Wednesday evening, a joint UK-France thanksgiving service was held at Normandy’s Bayeux Cathedral, which was illuminated in honor of those who fought on the beaches.
The King will travel to France for a commemorative event on Thursday at the British Normandy Memorial at Ver-sur-Mer, which will also be attended by the 23 veterans. The trip to France will be the King’s first overseas travel since his cancer diagnosis.
An international ceremony with more than 25 heads of state will be attended by Prince William.