The Queen led a commemoration of the Battle of the Somme at Westminster Abbey in a night-long vigil to remember one of the bloodiest battles in British Army history.
Prime Minister David Cameron also attended the service, and said it enabled people "to reflect on the sacrifice not just of the thousands of British and Commonwealth troops who gave their lives, but of the men on all sides who did not return home".
He added: "It is an opportunity to think about the impact of the devastation felt by communities across all of the nations involved, which left mothers without sons, wives without husbands and children without fathers."
The vigil is set to end at 7.30am on Friday - at the exact moment the whistles blew to signal the start of the battle and the Tommies went over the top in Picardy.
Mr Cameron is expected to join French president Francois Hollande and the Duke and Duchess of Cambridge at the Thiepval Memorial to the Missing on Friday, exactly a century on from the battle.
They will be joined by other heads of state and representatives from countries which fought in the battle, as well as descendants of soldiers, and around 10,000 members of the public.