Prince William and Kate Middleton led the official commemorations to mark 100 years since the Battle of Passchendaele began.
Prince William gave a speech at the Menin Gate monument in Ypres, Belgium.
William said Britain and Belgium "stand together" to remember those killed during weeks of heavy fighting in the summer and autumn of 1917.
"Today, the Menin Gate records almost 54,000 names of the men who did not return home; the missing with no known grave.
Members of our families; our regiments; our nations; all sacrificed everything for the lives we live today.
During the First World War Britain and Belgium stood shoulder to shoulder.
One hundred years on, we still stand together, gathering as so many do every night, in remembrance of that sacrifice."
The Menin Gate in the Belgian town has the names of 54,391 British dead who have no known grave, according to the Commonwealth War Graves Commission.