With a cannon blast and a piercing whistle, Britain and France on Friday marked 100 years since soldiers emerged from their trenches to begin one of the bloodiest battles of World War I at the River Somme.
Under grey skies, unlike the clear sunny day that saw the biggest slaughter in British military history a century ago, the commemoration kicked off at the deep Lochnagar crater, created by the blast of mines placed under German positions two minutes before the attack began at 7:30 am on July 1, 1916.
A lone piper walked around the edge of the crater at the ceremony, to be followed by a main event attended by the British royal family and Prime Minister David Cameron, as well as French President Francois Hollande and former German president Horst Koehler.
The commemoration ceremony will be held at the Thiepval Memorial, which honours more than 72,000 missing servicemen.
Just over a week after Britain's vote to leave the European Union, Hollande highlighted the friendship that saw British and French soldiers fight side by side.
"I want to recall that it is the European idea which allowed us to overcome divisions and rivalries between states, and which has brought us peace for the past 70 years," Hollande said in a statement before the ceremony.
Britain's Prince William on Thursday paid tribute to a generation lost at the Battle of the Somme at the start of an all-night vigil in memory of the Allied soldiers who would have been preparing themselves to charge the German side.
The following day some 20,000 soldiers were mowed down -- after a week of bombardment failed to destroy German defences -- in the deadliest day in British military history.
Another 30,000 were wounded and maimed.
"We lost the flower of a generation, and in the years to come it sometimes seemed that with them a sense of vital optimism had disappeared forever from British life," said William, attending alongside his wife Kate and brother Prince Harry.
"It was in many ways the saddest day in the long story of our nation."
The Battle of the Somme lasted 141 days, involving troops from across what was then the British Empire, and left around one million dead, injured or missing while moving the frontline only a few miles.
"Tonight we think of them... We acknowledge the failures of European governments, including our own, to prevent the catastrophe of world war," said Prince William.
The attritional battle became a defining event in the war, symbolising the horrors of trench warfare and the futility of the conflict.
"Imagine yourself, standing in a trench with water well over your knees... while thousands of unseen shells come shrieking and whining overhead. There is a very slight pause -- then it bursts and a tearing, rumbling blinding crash... hurling thousands of red-hot splinters in all directions, killing or maiming all they happen to strike," wrote Private Albert Atkins, who survived the war.
The ceremony at Thiepval is one of six in France, while Britain observed two minutes of silence to recall the moment when British, Commonwealth and French forces went "over the top" a century ago.
Then at 7:30 am sharp in Parliament Square in London, soldiers blew whistles -- the signal for the start of the offensive.
The observance came at the end of a night-long vigil in Westminster Abbey -- the first such vigil in the Abbey since the 1962 Cuban Missile Crisis -- which was attended by Queen Elizabeth II when it started on Thursday.
A century on, the eerily calm, bucolic Somme farmlands belie the slaughter wrought there, and are now the haunt of tourists visiting some 400 war cemeteries across the region, and the overgrown warrens of trenches.
Some 10,000 visitors are attending the commemoration ceremony under heavy security which has the rolling fields under lockdown, with only shuttles heading back and forth to the memorial.
"It's quite emotional in a way. I feel it's important to remember these things," 73-year-old Irishman William Vernon said on Thursday as he arrived for the ceremony.
Vernon said he was coming to remember his great-uncle -- also named William -- who died aged 26 in the battle.
Vernon's son, William, 33, said his forebear died "in the most horrendous conditions. It was an absolutely awful war, a pointless war. To be in the trenches was absolute torture," he said.