PASSCHENDAELE, Belgium (Reuters) - Relatives read letters and diary excerpts from soldiers who died in the World War One battle of Passchendaele when they gathered on Monday for a ceremony to mark 100 years since the start of one of the war’s bloodiest offensives.

Politicians and royalty joined relatives of the dead for the centenary of the 103-day battle in western Belgium in which more than half a million Allied and German troops were killed or wounded.

The brutal clash, which became a symbol of the horrors of the war, began on July 31, 1917 when Britain launched an assault against German forces who were holding the plateau overlooking the Belgium city of Ypres.

Hit by heavy rain, the campaign transformed the Flanders lowlands into a mud-churned swamp rendering tanks immobile and virtually paralyzing the infantry.

Some 4,000 relatives of the soldiers who fought there gathered at the Tyne Cot Cemetery near Ypres, where nearly 12,000 Commonwealth dead are buried.

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They were joined by Britain’s Prince Charles, Prince William, his wife Kate, Prime Minister Theresa May, and Belgian King and Queen Philippe and Mathilde.

At the service, Prince Charles honored the “courage and bravery” of those killed and said the gathering was “to promise we will never forget.”

Military personnel and descendants read out letters and diary excerpts penned by many of those who died. The ceremony concluded with a fly-past by the Belgian air force in a “missing man” formation.

Prince William paid tribute on Sunday at the Menin Gate Memorial in Ypres to those “who sacrificed everything for the lives we live today.”

As he spoke, thousands of paper poppies were dropped from the roof of the gate, which is engraved with the names of 54,391 British soldiers who have no known grave.