A service will be held later to mark 100 years since the Battle of Passchendaele – one of World War One’s bloodiest spells of fighting.
Prince Charles, Prime Minister Theresa May and the Duke and Duchess of Cambridge will attend the ceremony at Tyne Cot cemetery near Ypres.
They will be joined by 4,000 relatives of those who fought in the battle.
In the three months of fighting, half a million Allied and German soldiers were killed, wounded or went missing.
The conflict – officially known as the Third Battle of Ypres – was fought from 31 July 1917 until November that year.
It was not only infamous for the number of casualties but the mud. Many drowned in the thick quagmire, caused by weeks of relentless rain.
Descendants won tickets to the event at the cemetery in a ballot run by the government.
During the service, military personnel and descendants will read out letters and diaries from soldiers who fought at Passchendaele.
Catherine, Duchess of Cambridge, will lay flowers at the grave of the Unknown Soldier.
It will end with a fly past by the Belgian air force in a formation that pays tribute to “the missing man”.
At the scene: Thousands missing – but not forgotten
By Kate Palmer, BBC News, at Tyne Cot cemetery
Once a battlefield of liquid mud, Tyne Cot is now an immaculately maintained cemetery.
Thousands of people have gathered for Monday’s service to mark the centenary of the Battle of Passchendaele, but their numbers are dwarfed by rows of pristine graves and an imposing stone Memorial to the Missing.
“It has hit home, quite how disastrous the death toll was,” says Ann Philips, whose uncle Edward Woolley was killed weeks into the battle, age 22, on 22 August 1917.
She is wearing a white dress with poppies, as she finds her Uncle Ted’s name on the cemetery’s memorial, alongside 35,000 other missing soldiers.
Many were a similar age to student Daniel Fay, 20, whose great-great-uncle James McBarrons was a 28-year-old labourer in Paisley before the war – but never returned.
“It makes me think of my group of friends who would have been the same age,” he says.
A few miles away in Ypres, the city has hosted a weekend of culture, telling stories of heroism and sacrifice in song and plays. But today strikes a solemn, more contemplative tone.
Other events were held on Sunday in Ypres to mark the centenary, including a reading by War Horse author Michael Morpurgo.
The duke and duchess began commemorations by joining the Belgian king in paying tribute to the fallen at the Menin Gate in Ypres.
The Last Post was played at the end of the service at the memorial, where the names of 54,000 missing soldiers are inscribed. A bugler has played the tune at the gate almost every evening since 1928.
Images from the war and recordings of interviews with World War One veterans were projected on to the town’s Cloth Hall.
Mrs May said she was “honoured” to attend the event, where she will lay a wreath alongside representatives of the combatant nations.
She 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.