The king has paid tribute to D-day veterans at a commemorative event in Portsmouth marking the 80th anniversary of the Normandy landings.

Charles said their “stories of courage, resilience and solidarity” move, inspire and “remind us of what we owe to that great wartime generation – now, tragically, dwindling to so few”.

Royals, politicians, senior military leaders and veterans began two days of remembrance events in Portsmouth and in Normandy, northern France to mark the largest seaborne invasion in history, which laid the foundation for an allied victory in the second world war.

It was from Portsmouth on 5 June 1944 that the many of the allied troops set forth as part of Operation Overlord to attack German forces on the beaches of Normandy on 6 June.

The king said: “Those who gathered here in Portsmouth would never forget the sight. It was by far the largest military fleet the world has ever known. Yet all knew that both victory and failure were possible, and none could know their fate.

“Aircrew flying overhead, sailors manning warships; or troops in assault craft battering their way through the stormy swell to the shore; whether dropping by parachute, landing in a wooden glider, or taking that terrible leap of faith on to the beaches … all must have questioned whether they would survive and how they would respond when faced with such mortal danger.”

He added: “At this remove, eight decades later, it is a near impossible task to imagine the emotion of that day: the pride of being part of so great an enterprise, the anxiety of in some way not coming up to scratch, and the fear of that day being their last.

“It is our duty to ensure that we, and future generations, do not forget their service and their sacrifice in replacing tyranny with freedom. Our rights, and the liberty won at such terrible cost, bring with them responsibilities to others in the exercise of that liberty.”

Of all those who fought, and all those who died, he added: “We are all, eternally, in their debt.”

The host, Dame Helen Mirren, introduced the king and queen on stage. The commemorations opened with a musical performance and a flypast of two historic Dakota military transport aircraft, widely used by the allies during the second world war. The D-day veterans joined in singing wartime favourites such as Dame Vera Lynn’s We’ll Meet Again.

Prince William took his place in the royal box alongside military veterans shortly after the prime minister, Rishi Sunak, and the Labour leader, Sir Keir Starmer.

William read an extract from the diary of Capt Alastair Bannerman of the Royal Warwickshire regiment. Then, addressing the crowd, he said: “We will always remember those who served and those who waved them off. The mothers and fathers, brothers and sisters, sons and daughters who watched their loved ones go into battle, unsure if they would ever return.”

The prime minister read an address by Field Marshal Montgomery, which was delivered to the troops before the D-day landings, which began: “The time has come to deal the enemy a terrific blow in western Europe.” In the commemorative programme, Sunak wrote: “We must never forget how much we owe.”

The first sea lord and head of the Royal Navy, Adm Sir Ben Key, said of the veterans: “They are phenomenal, the way they tell their stories, the humility, the humour. They clearly reflect that there are so few of them left now.

“You can see in their eyes that they recall those of their mates who are no longer here, some of whom lost their lives on the very first day of the landings,” he told the PA news agency.

The chief of the defence staff, Adm Sir Tony Radakin, said: It’s quite emotional, it’s just to reflect that these are old people, to reflect on most soldiers that were going across had never been in combat before, the average age was in the mid-20s.”

The Portsmouth commemorations concluded with a gun salute from the frigate HMS St Albans, which sailed past in the Solent, and the national anthem, before a flypast by the Red Arrows in formation, trailing the team’s trademark red, white and blue colours, and Typhoon jets. Charles, Queen Camilla, William and Sunak went on to meet the veterans at a private reception after the event.

There will be further commemorations at the Bayeux war cemetery and a joint UK-France thanksgiving service at Bayeux Cathedral.

On Thursday commemorations will begin in Normandy at 7.25am, the same time the beach invasion began in 1944. A military piper will land on the beaches of Arromanches-les-Bains in a Royal Marines landing craft and begin playing a lament in tribute to those who led the beach landings.

The official British commemoration on the 80th anniversary of D-day will take place at the British Normandy Memorial at Ver-sur-Mer, where the king will join the French president, Emmanuel Macron, and Sunak.

Prince William will attend events in Normandy including a service hosted by Canada at Juno beach and an international ceremony hosted by France at Omaha beach, which will be attended by more than 25 heads of state.