The Duke and Duchess of Cambridge will join Prince Harry at the Tower of London today to each plant a ceramic poppy in an art installation symbolising the thousands of lives lost during the First World War.

The visit to the exhibition, entitled Blood Swept Lands And Seas Of Red, at the Tower's moat comes a day after the Royals joined political leaders and relatives of the fallen in Belgium to remember the sacrifices and losses exactly a century on from Britain's entry into the war.

A moving twilight ceremony at St Symphorien military cemetery near Mons was the highlight of a day of events in the UK and Belgium marking 100 years since the Great War's start.

Today William, Kate and Harry will visit the dry moat at the Tower - the site where more than 1,600 men swore an oath to the crown in August 1914 after enlisting for war.

The first ceramic poppy installed as part of the Historic Royal Palaces artwork was planted last month, and the final will be laid on Armistice Day, November 11.

In total 888,246 ceramic poppies, one for each British and Colonial death during the war, will be installed by a team of 8,000 volunteers.

Profits from the artwork will be divided between six service charities including Help for Heroes and the Royal British Legion.

The royals stood alongside Prime Minister David Cameron and counterparts from countries including France and German at commemorative events yesterday.

Mr Cameron said the Great War, which claimed millions of lives, including 750,000 from the British and Commonwealth, was "unlike any other".

Harry read a letter from Private Michael Lennon, of 1st Battalion the Royal Dublin Fusiliers, dated May 30 1915, who wrote to his brother Frank the day before he was due to land in Gallipoli.

He was killed in action on June 28 1915 - exactly a year after Archduke Franz Ferdinand was assassinated in Sarajevo, sparking the war.

In the letter, Pte Lennon wrote: "Well Frank, I suppose we are for it tomorrow, if we don't get shelled on the way."

Performances were heard from a children's choir conducted by Gareth Malone, as well as a recording of a collaboration between the London Symphony Orchestra and the Berlin Philharmonic Orchestra.

As the ceremony - narrated by TV historian Dan Snow - moved to a close, William, King Philippe of Belgium, Mr Cameron, and the German and Irish presidents laid flowers at the obelisk.

The Last Post was sounded, followed by a minute's silence and the reveille, then a lone piper played, after which each of the dignitaries placed a lantern on the obelisk.

St Symphorien is where 229 Commonwealth and 284 German troops are buried, including the first and last British soldiers to die on the Western Front, and the recipient of the first VC of the war.

Before the ceremony, William, Kate, Harry and Mr Cameron attended a reception for guests and met relatives of those buried at the cemetery.

Earlier, William spoke at the Allies' Memorial at Cointe and said: "We were enemies more than once in the last century, and today we are friends and allies."

William and Kate were in Belgium representing the Queen, who attended a service of commemoration in Crathie Kirk in Crathie, Aberdeenshire, near Balmoral.

At 11pm on August 4 1914, Britain declared war on Germany, ushering in four years of darkness, despair and appalling tragedy.

Until the armistice was signed on November 11 1918, soldiers engaged in the bloodiest conflict the world had known.

In the UK to mark its beginning, the Prince of Wales attended a service at Glasgow Cathedral, which was followed by a wreath-laying service and marchpast at the Cenotaph in George Square.

Later Britain was plunged into darkness as lights were switched off for an hour across the country to conclude the day of ceremonies.

At the same time, the Duchess of Cornwall joined senior politicians for a service of solemn commemoration at Westminster Abbey.

Key figures present included Deputy Prime Minister Nick Clegg, former foreign secretary William Hague, Labour leader Ed Miliband and Metropolitan Police commander Sir Bernard Hogan-Howe.

The service included the gradual extinguishing of candles, with an oil lamp snuffed out at the grave of the unknown warrior at 11pm - the exact hour war was declared.

In Afghanistan, personnel from all three services were joined by the US Marine Corps at Camp Bastion to mark the occasion.

Around 400 personnel congregated at the base's Vigil Site for a parade illuminated by the lights of military vehicles and the site itself, before a ceremony presided over by force senior chaplain Wing Commander Geoffrey Withers.