The centenary of the end of one of the bloodiest episodes of the first world war, the 141 days of the Battle of the Somme in which more than a million men on both sides were killed or injured, is being marked with services in France and in the UK.

At the monument in Thiepval – which has been inscribed with the names of 72,000 men whose bodies were never identified – guests including politicians, representatives of the armed forces and descendants of those who fought in the battle are gathering for a service led by Bishop James Newcome, the national chaplain of the Royal British Legion.

A daily service has been held at the monument since the ceremonies marking the first day of the battle, 1 July, on which almost 20,000 British soldiers died, the single bloodiest day in the history of the British army. At the end of the 141 days of trench warfare that came to symbolise the war, the British had taken just seven miles of land.

A restored print of the contemporary silent film of the battle, which was billed in 1916 as “the most remarkable film ever shown to the public” and watched by 20 million people, has been on tour since July. It features live orchestras performing a new score by Laura Rossi in venues including concert halls, arts centres, cathedrals and the Twickenham rugby stadium. The centenary of the end of the battle is marked with a screening at Royal Festival Hall in London, and the tour continues until July 2017.

Many local ceremonies are also being held, including one in County Down for a local man, L/Cpl William Samuel Montgomery, who joined in December 1915, and was worried the fighting on the western font would be over before he could get there.

He was repatriated gravely wounded and died after an operation in a military hospital in Sheffield, hours before the official end of the battle. His last letter to his sister asked her to send him the homely Northern Ireland foodstuffs he craved: soda bread, potato cakes and buttermilk. The Last Post will be played and a poppy wreath laid on his grave in Killyleagh.