The WWI battle lasted 141 days, and left more than 1 million dead or wounded on all sides. Britain's Prince William, his wife Kate, and brother Prince Harry have attended an all-night vigil in Thiepval, France.
Britain's Prince William paid tribute to the tens of thousands of British soldiers who perished in the Battle of the Somme, one of the bloodiest chapters of World War I.
On the deadliest day of the deadliest battle in British history, 20,000 soldiers died, and tens of thousands more were wounded, some of them for life.
"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 for ever from British life," William said at a ceremony in northern France.
A soldier at a military-led vigil to commemorate the 100th anniversary of the beginning of the Battle of the Somme
""It was in many ways the saddest day in the long story of our nation," he added.
William, his wife Kate and brother, Prince Harry attended the start of an all-night vigil at the Thiepval Memorial to honor the 1.2 million troops of different nationalities who were killed, injured or listed as missing.
Soldiers from the United Kingdom, France, Australia, Canada, Germany, India, Ireland, New Zealand and Pakistan are taking turns maintaining the overnight vigil until 7:30 a.m. (0530 UTC), the precise time that the battle began.
Tens of thousands of troops were mowed down by German guns that first day, as they tried to advance from their trenches.
"Tonight we think of them... We acknowledge the failures of European governments, including our own, to prevent the catastrophe of world war," the prince, also known as the Duke of Cambridge, said.
William Vernon, a 73-year-old Irishman, came to attend the main commemoration ceremony on Friday
"It's quite emotional in a way," he said. "I feel it's important to remember these things."
Vernon came to remember his great-uncle - also named William - who died in the battle at the age of 26.
His 33-year-old son, also William, said it's important to honor to his relative who died "in the most horrendous conditions," he said. "It was an absolutely awful war, a pointless war. To be in the trenches was absolute torture."
In London, Queen Elizabeth II attended a service at Westminster Abbey Thursday evening, and laid a wreath of roses on the grave of the Unknown Warrior.
Allied forces launched the battle with the aim of relieving the pressure on French forces - who were absorbing heavy casualties in Verdun - by attacking the Germans further north at Somme.
But carnage ensued as neither side could move the frontline. The battle ground on for 141 days, and ended with more than 1 million dead or wounded. The fight came to symbolize the horrors of trench warfare and the futility of the conflict.
bik/bw (AFP, AP)