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 in 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.
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."
The conflict between 1914 and 1918 - which became known as the Great War - left 17 million soldiers and civilians dead.
Tower Bridge, the Eden Project in Cornwall, the headquarters of the Football Association and the Imperial War Museums in London and Greater Manchester, were among the buildings which took part in the "lights out" event.
The Duchess of Cornwall joined senior politicians - including Deputy Prime Minister Nick Clegg and Labour leader Ed Miliband - for a service of solemn commemoration at Westminster Abbey.
The service included the gradual extinguishing of candles, with an oil lamp put out at the grave of the unknown warrior at the exact hour war was declared.
The day's events began in Liege where 50 heads of state gathered for a service to mark the invasion of Belgium - which led to Britain declaring war in 1914.
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.
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 honour of the British dead, in a ceremony that was beautifully conceived and executed.
(BBC)
