Princes Charles, the Duke and Duchess of Cambridge, dignitaries and hundreds of guests paid their respect for the more than half-a-million Allied and German troops that were killed, injured or missing following the Third Battle of Ypres.
The battle - better known as Passchendaele - began on this day in 1917 and raged for almost four months. It went on to become a symbol of the conflict's futility.
The head of Remembrance of the Royal British Legion, the Rt Revd Nigel McColloch, told Premier that even though it happened 100 years ago, it still carries great significance on decisions made today.
He said: "One hopes really that this reminder of Passchendaele - this most ghastly battle, will help people of a new generation to realise how terrible war really is and how it must be avoided at all costs."
The Prime Minister, defence secretary Sir Michael Fallon and Belgian king and queen Philippe also attended the event.
Monday's commemoration centred on the Tyne Cot cemetery near Ypres in Belgium, which is the largest Commonwealth burial ground in the world with 11,971 servicemen buried or remembered there - with 8,373 of them identified.
Dignitaries and guests watched the show, led by Dame Helen Mirren, which included testimony from soldiers projected on to the walls of the Cloth Hall.
It wasn't just Brits that marked today's anniversary.
People from Belgian also paid their respects at the official memorial.
Speaking about the fallen soldiers, Rt Revd McColloch told Premier: "They were determined to do what they could for their sovereign and country. And when I say 'they' we have to remember this is not only British people from the United Kingdom, this is British people from across the empire.
"A huge number of Indian soldiers, people from the West Indies, East Africa - even China and Egypt. I would imagine that if you were there in those terrible days, you really would have understood that the world was war."
