The Prince Of Wales has laid a wreath in northern France to mark the 90th anniversary of the Battle of Somme.
Prince Charles greeted 110-year-old war veteran Henry Allingham
More than 19,000 British soldiers died on the first day of the battle - the British Army's worst day - and 125,000 died over the next five months.
Around 5,000 people gathered in front of the soaring Thiepval Monument to take part in the commemoration.
The prince described the battle as "unutterable hell" and said "mere boys" fought with "deep courage".
"The magnitude of the allied losses on July 1, 1916 are unimaginable in these days of instant communication and ever-present media," said the prince, who was accompanied by his wife, the Duchess of Cornwall.
"But even 90 years ago they caused a most profound shock to our nations and left scars that remain with us today.
"Being here today can only go a very small part of the way in helping us imagine how this beautiful countryside was devastated."
Whistles were blown to signal the start of the attack - just as they were in 1916.
Over the five months of the battle more than a million soldiers were killed or wounded.
The figure included German and French troops, as well as Australians, Canadians, New Zealanders and Irishmen who were fighting with the British forces.
The commemorations took place in bright sunshine, in contrast to the torrential rain which dogged the later stages of the battle.
Defence Secretary Des Browne, in northern France, said: "I can't look at a row of gravestones that are there to remember people, some of them very young, some of them in their teens, without a great sense of sadness."
He added: "But you're greatly humbled by it as well, because these people gave selflessly of their own lives in order to protect our freedom."
The Battle of the Somme left more than a million killed or injured
There are no known UK veterans of the Somme alive but Britain's oldest war veteran, 110-year-old Henry Allingham who served in the Royal Naval Air Service and RAF in World War I, met the Prince of Wales at the commemorations.
Remembering the dead
As well as formal commemorations at Thiepval, smaller gatherings also took place along the 18-mile stretch of the front.
Jean Matthews, 73, from Cambridge, whose uncle Alec Law survived the Somme, was at a service in France with three generations of her family.
She said: "We bring our friends year after year, we always bring a car full of people, and this year we wanted my grandson to come."
Keeping the memories
Mrs Matthews' daughter, Cheryl Cockle, added: "Ninety years is obviously a long time ago and there are no people with first-hand memories.
"It seems to be something that will unfortunately slip away but you have to make sure it doesn't."
Charles and the Duchess of Cornwall visited war graves
In 1916, thousands of men were sent over the top in an attempt to break the stalemate with the Germans after 18 months of trench warfare.
After the service, which included a fly-past by two WWI planes, the prince and the Duchess of Cornwall visited war graves, where 300 French and 300 British fighters are buried.
The Duchess lay posies at the graves of two unknown soldiers.
The Princess Royal, who is Colonel-in-Chief of the Royal Newfoundland Regiment, attended a Canadian ceremony at Beaumont Hamel on the Somme.
Meanwhile, the Duke of Gloucester, who is honorary president of the Somme Association, was at Ulster Tower, the oldest official memorial on the western front dedicated to the 36 (Ulster) Division and other Irish soldiers.
In Ireland, the government commemorated soldiers who fought and died with the British at the Somme, for the first time, with a ceremony in Islandbridge.
Some 70,000 Irishmen died during WWI, but political sensitivities meant their contribution had not been recognised at home until now.
Irish President Mary McAleese laid a wreath during the historic service at the Irish National War Memorial Gardens.
The Irish Prime Minister Bertie Ahern and the Lord Mayor were among 800 dignitaries who attended the service.
Commemorations are also being held around the UK, and a wreath has been laid at the Tomb of the Unknown Warrior in Westminster Abbey.