British and Belgian royals led tributes Sunday to the soldiers who fell at Passchendaele as they marked the centenary of one of World War I's bloodiest and most senseless battles.

Three months of fighting near the west Flanders village of Passchendaele claimed around half a million allied and German casualties, many of them disappearing into the mud forever.

Prince William, second in line to the throne, and his wife Kate, along with British Prime Minister Theresa May, joined King Philippe of Belgium and his wife Mathilde for two days of ceremonies marking the battle that began on July 31, 1917.

"Members of our families, our regiments, our nations, all sacrificed everything for the lives we live today," said Prince William, a red poppy on the lapel of his dark blue suit.

"During the First World War Britain and Belgium stood shoulder to shoulder," he said as around 200 descendants of the combatants looked on.

"One hundred years on, we still stand together, gathering as so many do every night, in remembrance of that sacrifice."

William and King Philippe laid wreathes at the Menin Gate, the monument which honours the dead of the armies of the British empire "who stood here ... and who have no known grave".

Some 55,000 names are inscribed on its great limestone walls, where visitors leave small wooden crosses adorned with a red poppy, or a wreath and a few words of gratitude and comfort, 100 years later.

The ceremony ended with the last post, a lament to the dead played on a bugle.

First begun as a gesture of thanks to the allies by the town in 1928, the ceremony has pretty much continued every night since.

The exception was during World War II -- yet another conflict whose dead lie here.

- 'Passchendaele resonates' -

In a statement before the event, May said: "The name Passchendaele resonates with anyone with even a passing knowledge of the First World War.

"It is on those fields where hundreds of thousands of men of all nations fought and died in appalling conditions," she added.

The battle of Passchendaele that lasted until November 6, 1917 is known as the third battle of Ypres, a town which is synonymous with the bloody stalemate that World War I had become.

The aim was to drive the Germans from the Belgian ports on the English Channel, where German U-boats lurked.

In the end, the Commonwealth troops advanced only five miles (eight kilometres), albeit weakening German defences.

On Monday, thousands of descendants and others with a connection to the battle are due to turn up at the Commonwealth War Graves Commission's Tyne Cot Cemetery, where 11,961 men are buried.

Serving military personnel and descendants are expected to read out letters and diaries from their ancestors in tribute to those who fought at Passchendaele.

Tyne Cot only counts a small fraction of the total casualties, put at some 245,000 British dead and wounded and perhaps 270,000 for the Germans in this single battle.

Source: AFP