Media playback is unsupported on your device Media caption Lights have been turned off around the UK in memory of those who died in World War One

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.

Image copyright PA Image caption A candle-lit vigil was held at Westminster Abbey

Image copyright Reuters Image caption A single lantern flickered outside Downing Street

Image copyright PA Image caption The Houses of Parliament were plunged into darkness

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.

'Deadly days'

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.

Media playback is unsupported on your device Media caption Some of the highlights of WW1 centenary events in the UK, Belgium and Singapore

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.

Image caption The Last Post was played at St Symphorien Military Cemetery near Mons

Image caption Prince William said European countries which had fought bloody wars were now "friends and allies"

Image copyright Getty Images Image caption David Cameron gave a Bible reading at Glasgow Cathedral earlier in a service attended by Prince Charles

Image copyright Getty Images Image caption Prince Harry attended the unveiling of a memorial arch in Folkestone

Image caption A white rose was laid at the Mons grave of the first British soldier to die in western Europe, John Parr

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

Image copyright PA

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.

'Unspeakable carnage'

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

Image copyright AP

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.

Ceramic poppies

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:

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.

Image copyright Getty Images Image caption A re-enactment group dressed as Gordon Highlanders held an event at the Tank Museum in Bovington, Dorset

Image copyright EPA Image caption King Philippe of Belgium hosted a ceremony in Liege attended by more than 50 heads of state

Image copyright PA Image caption Prince Charles laid a wreath at the Cenotaph in George Square, Glasgow

Image copyright Getty/Corp Daniel Wiepen RLC Image caption A photo of men of the British Machine Gun Corps in the Battle of the Somme (left) was recreated by the Queen's Dragoon Guards in Afghanistan

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

Image copyright PA

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."

Media playback is unsupported on your device Media caption David Cameron said it was right to commemorate WW1

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.

Image copyright Getty Images Image caption Britain's entry into WW1 was announced at 23:00 on 4 August 1914

Image copyright Getty Images Image caption Ceramic poppies will be placed around the Tower of London throughout the summer until there are 888,246