A candle-lit vigil at Westminster Abbey and a lights out event have concluded a day of ceremonies marking 100 years since Britain entered World War One. People were invited to turn off their lights for an hour until 23:00 BST, the time war was declared on 4 August 1914.
The Duke and Duchess of Cambridge, Prince Harry and David Cameron attended a twilight ceremony at St Symphorien Military Cemetery near Mons, Belgium. The Prince of Wales was at a service in Glasgow, among other commemorations.
About 17 million soldiers and civilians were killed between 1914 and 1918.
The Royal British Legion's Lights Out event saw households, businesses and public buildings across the UK turn out their lights to leave a single candle or light burning.
The event was inspired by the words of wartime Foreign Secretary Sir Edward Grey, who said on the eve of WW1: The lamps are going out all over Europe; we shall not see them lit again in our lifetime.
Tower Bridge, the Eden Project in Cornwall, the headquarters of the Football Association and the Imperial War Museums in London and Trafford, Greater Manchester, were among the buildings which took part in the event.
As part of a remembrance project called the Big Picture, giant images of candles were projected on to buildings including Battersea Power Station in London and the Royal Liver Building in Liverpool during lights out hour.
The day's events began in Liege where 50 heads of state gathered for a service to mark the invasion of Belgium.
French President Francois Hollande said the country had been the first battleground of WW1 and had offered solid resistance in Liege.
Deadly days followed when French and British soldiers joined the conflict, he said.
Speaking to the gathered European leaders, Prince William said: We were enemies more than once in the last century and today we are friends and allies.
We salute those who died to give us our freedom. We will remember them.
St Symphorien Military Cemetery is unique: opened by the Germans in 1917, taken over by the British after the war, it holds more than 500 graves, roughly half German and half British and Commonwealth.
It combines the white gravestones and manicured lawns familiar from countless British military cemeteries with the dark stone and woodland glades of their German equivalents.
What's more it is on the outskirts of Mons, where British and German armies first clashed in a battle quite unlike the muddy trench warfare of the next four years, an affair of cavalry charges, infantry advances over fields at harvest time and artillery deploying among factories and coal mines - before the British were forced into a 200-mile, two-week-long retreat.
It was the perfect venue for what was billed as an event of reconciliation. Princes and politicians, soldiers and civilians came together to remember - enemies a century ago, allies now.
They read from the letters and diaries of those who had fought and died. Musicians from the London Symphony Orchestra and the Berlin Philharmonic under Sir Simon Rattle played Brahms' German Requiem and the music of George Butterworth, killed on the Somme.
And as dusk fell they laid wreaths at the foot of an obelisk among the trees erected by the Germans in honor of the British dead, in a ceremony that was beautifully conceived and executed.
Meanwhile, the service at Glasgow Cathedral, attended by Prince Charles, UK Prime Minister David Cameron, Scotland's First Minister Alex Salmond and 1,400 others, heard from a number of Commonwealth figures.
Prince Harry unveiled a memorial arch in Folkestone, Kent, where a parade followed the route taken by millions of soldiers who marched to the harbor to begin their voyage to France in WW1.
The first major battle between British and German forces in the war was fought at Mons.
In an address at St Symphorien, Mr Cameron said: Every war is cruel but this war was unlike any other: the unspeakable carnage, the unbearable loss, the almost unbelievable bravery.
One hundred years on, it is right that we meet here and around the world to remember.
Prince Harry read out the words of a letter from Pte Michael Lennon, of the Royal Dublin Fusiliers - sent to the soldier's brother Frank in 1915 - days before he was killed at Gallipoli.
The beautiful military cemetery at St Symphorien has been transformed.
Across the boundary fence, in what is normally empty farmland, a great grandstand has risen, overlooking the graves of the first and the last British soldier to die in World War One.
Cables snake around the gravestones of British and German soldiers, laid here side by side after the battles that raged around Mons on summer days in 1914.
St. Symphorien has become an arena, overlooked by a worldwide audience, where Monday evening's televised event will mark personal sacrifice and celebrate new friendships. (BBC).-