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 Lights Out event - organised by 14-18 NOW, a cultural programme to mark the centenary - 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.
Blackpool Tower, Downing Street, 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.
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."
At the scene
Nick Higham, BBC News, Mons
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.
A twilight ceremony, held at St Symphorien military cemetery, near Mons, was also attended by the Duke and Duchess of Cambridge, Prince Harry and UK Prime Minister Mr Cameron, as well as counterparts from France and Germany.
The cemetery remains of particular significance as an equal number of German and British soldiers are buried there, including Private John Parr, the first British soldier killed on the Western Front and Irishman Maurice James Dease, who was the first recipient of the Victoria Cross in WW1.
In an address, 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.
At the scene
Robert Hall, BBC News, Mons
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.
Under the pine trees, David MacCarthy had come to find the grave of his great-uncle, killed 10 days after arriving in Belgium, aged 23.
Standing in front of the headstone with his daughter, Mr MacCarthy said he was proud to be here on this anniversary.
The families who have travelled here from Britain and Germany share those sentiments.
The duke and duchess earlier made a "very private" visit to see graves of British, Commonwealth and German soldiers at St Symphorien cemetery.
William and Catherine were then greeted by crowds when they arrived at Mons Town Hall.
Other commemorations included:
- A wreath-laying ceremony at the Cenotaph in Glasgow - one of many events being held in Scotland
- In Northern Ireland, a service at St Anne's Cathedral in Belfast was attended by First Minister Peter Robinson and NI Secretary Theresa Villiers
- In Wales, a national service of remembrance at Llandaff Cathedral in Cardiff took place from 22:00
- In Dundee, a war memorial time capsule put together by postal workers in 1921 was opened
- A new memorial was unveiled in Bramley, Leeds, in honour of 500 fallen soldiers from the area
- All Saints Church in Dunsden, Oxfordshire, where World War One poet Wilfred Owen worked, put his poems to music
In Scotland, a service was held at Glasgow Cathedral - attended by Prince Charles, Mr Cameron, Scotland's First Minister Alex Salmond, a number of Commonwealth figures and 1,400 others.
Prince Harry also unveiled a memorial arch in Folkestone, Kent, where a parade followed the route taken by millions of soldiers who marched to the harbour to begin their voyage to France in WW1.
Elsewhere, 888,246 ceramic poppies are being placed in the dry moat at the Tower of London, one for each soldier who died fighting for Britain and its colonies in WW1.
The installation by artist Paul Cummins is entitled Blood Swept Lands and Seas of Red and will be unveiled on Tuesday.
At the scene
Laura Bicker, BBC News, Glasgow
Last night they gathered to hear Hampden roar for the closing ceremony of the Commonwealth Games.
Today the leaders and representatives from across the Commonwealth stood in silence at Glasgow's medieval cathedral. They were given a single poppy for their own personal act of remembrance.
Officers representing the armed forces walked through the aisles next to a child. Some were from the local Sunday school or the Scouts and Brownies. They held a candle of peace and hope. An act of remembrance to be taken on by the next generation.
And it was 16-year-old Kirsten Fell from Dunbar who had the final word. She read an essay written after a school trip to Flanders where she had been placed in front of a headstone of an unknown soldier and told to imagine his life.
"They told us we would change," she told the congregation. "It will always be with me and nothing will be forgotten. I will remember my soldier."
The Queen did not attend any of the major ceremonies but paid her respects at a service in Crathie Kirk near Balmoral in Scotland.
On Sunday, the French and German presidents commemorated the 100th anniversary of Germany's declaration of war on France on 3 August 1914.