Video: How the lamps went out over Yorkshire last night

The Prime Minister joined the Duke and Duchess of Cambridge and Prince Harry at St Symphorien military cemetery near Mons, Belgium, as part of events across the UK and Belgium to mark the centenary of the start of the war, which culminated when people across the nation switched off their lights as an act of commemoration.

Around 500 guests attended the event in Belgium including relatives of those laid to rest at St Symphorien, where 229 Commonwealth and 284 German troops are buried, among them the first and last British soldiers to die on the Western Front, and the recipient of the war’s first VC.

Within weeks of Britain declaring war on Germany, the two nations’ forces clashed outside Mons.

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.”

He said: “Its legacy still affects us today – good and bad.

“Too often it has been dismissed as a pointless war, fought by people who didn’t know why they were fighting.

“But that is wrong. These men signed up to prevent the domination of a continent, to preserve the principles of freedom and sovereignty that we cherish today.”

He added: “We should never fail to cherish the peace between these nations and never underestimate the patient work it has taken to build that peace.

“So, 100 years on, it is right that collectively we stop, we pause; and we re-pledge this for the next 100 years.

“We will never forget. We will always remember them.”

The woodland cemetery, which is maintained by the Commonwealth War Graves Commission, was lit in the dusk by spotlights as the daylight faded.

The service included readings, music and poetry capturing the unique history of the site, and aimed to acknowledge the British, Irish, Commonwealth and German war dead.

A letter to the War Office from the mother of John Parr, who is believed to be the first British soldier to have died on the Western Front and is buried at St Symphorien, was read by his grand-niece Iris Hunt.

She held a single white rose as she delivered the reading next to his grave which she kissed as she laid it next to his headstone.

Earlier, at a speech in Liege, the Duke of Cambridge said war betwen the nations of Europe was now “unthinkable” but he warned events in Ukraine were testament to the fact “instability continues to stalk our continent”.

He said: “We were enemies more than once in the last century, and today we are friends and allies.”

German president Joachim Gauck said it was “unjustifiable” for Germany to have invaded Belgium. He added: “We are grateful to have been able to live together with peace for so long in Europe.”

At 10pm last night – an hour before war was officially declared 100 years ago – a service of solemn commemoration was held at Westminster Abbey.

The service included the gradual extinguishing of candles, with an oil lamp extinguished at the tomb of the unknown soldier at 11pm – the exact hour war was declared.

In the same hour, the nation switched off lights to leave one light burning as a symbol of hope in darkness, in a reference to then-foreign secretary Sir Edward Grey’s famous remark that the “lamps are going out all over Europe”.