Commemorations have taken place in northern France to mark the 90th anniversary of the Battle of the Somme.
The Somme battlefields claimed more than one million casualties
Some 20,000 troops of the British Army fell on the first day as they advanced on German positions during World War I.
Germany, France, Ireland, New Zealand, Australia and Canada also suffered huge losses in the five-month battle, which claimed more than a million casualties.
At 0730 local time, whistles were blown in Thiepval to signal the start of the attack - as they were 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.
Correspondents say the slaughter of the first day is still remembered as a watershed marking the sacrifice and suffering of a generation.
On Saturday morning, a mock explosion marked the moment when mines were detonated underneath German positions and British and Irish troops began the battle.
There are no known veterans of the battle still alive
Church bells rang out as the sun burned mist off the green fields that saw such slaughter.
There are no known veterans of the battle still alive, but families have been attending remembrance services.
Hundreds gathered for prayers in English, French and German at one service near the village of La Boisselle.
Thousands of poppy petals were scattered into a crater at the site.
Dignitaries assembled under the great arch of the memorial to the dead in Thiepval.
Prince Charles was among those at the commemoration, which marks the British Army's worst day.
"For the first time in our history, we put mere boys into an assault against the bomb, bullets and the terrible wire entanglements, equipped with little more than raw courage," the prince said.
"It is impossible not to be overwhelmed by a mix of deep emotion, humiliation and awe, sadness and pride."
Across the fields, Canadians, New Zealanders, French, South Africans and Australians have also been remembering their dead.
Britain led Allied forces into battle on 1 July in an effort to end the deadlock with a decisive victory over German forces.
However, unconcealed preparations for the assault and a week-long bombardment gave the Germans clear warning. They moved underground until the shelling ended and then set up their positions.
When the British climbed out of their trenches that morning, they faced a hail of machine gun fire.
By the end of the day, the British had suffered 60,000 casualties.
Torrential rains in October turned the battlegrounds into a muddy quagmire and in mid-November the battle ended, with the Allies having advanced only 8km (five miles).
The British suffered around 420,000 casualties, the French 195,000 and the Germans around 650,000.