LONDON (AFP).- Lights went out across Britain on Monday to commemorate 100 years since the outbreak of World War I, which claimed the lives of nearly one million Britons.

Homes, businesses and famous landmarks turned off their lights for an hour from 10:00 pm (2100 GMT) and lit a single candle, inspired by a remark by Britain's then foreign minister Edward Grey on the eve of war that "the lamps are going out all over Europe; we shall not see them lit again in our lifetime."

A lantern was left on the steps of 10 Downing Street, the official residence of Prime Minister David Cameron, as part of the campaign organised by the Royal British Legion charity.

And a tower of white light shone straight into the sky from central London, an art installation by Japan's Ryoji Ikeda described as a "unifying point" to mark the anniversary by city mayor Boris Johnson.

At the nearby Tower of London, a striking art installation made up of hundreds of thousands of ceramic poppies -- one for each British soldier killed in the war -- poured out of the tower into the surrounding moat.

Cameron called on people to "never fail to cherish" peace at an evening ceremony in a military cemetery near Mons, Belgium.

"Here on the continent of Europe we saw not the war to end all wars, but the precursor to another desperate and violent conflict just two decades later," Cameron said.

"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 ceremony was attended by several hundred people including descendants of the soldiers buried in the cemetery, which contains the graves of both German and Commonwealth soldiers.

The site was allowed as a burial ground by its owner on the condition that both British and German soldiers were buried with equal dignity.

The ceremony was attended by British royals Prince William and his wife Catherine and Prince Harry, as well as the King and Queen of Belgium, Germany president Joachim Gauck, Irish president Michael D. Higgins and Archbishop of Canterbury Justin Welby.

The Duchess of Cambridge, dressed in a cream coat and floral hat, laid a bunch of flowers during the ceremony, and the Last Post was played by a soldier as darkness fell.

Queen Elizabeth II attended a commemoration at Crathie Kirk, the small Scottish church used by the royal family when holidaying at nearby Balmoral Castle.

Charles's wife Camilla attend a service at London's Westminster Abbey.

Elsewhere, the famous red-coated Chelsea pensioners, former members of the British Army, took part in a procession of more than 40 Edwardian cars through central London.

New risk of complacency
Britain declared war on Germany at 11 pm on August 4, 1914, after it rejected an ultimatum issued following the invasion of neutral Belgium.

Future wartime leader Winston Churchill described the mood in London as war was declared in his diary.

"It was 11 o'clock at night -- 12 by German time -- when the ultimatum expired," he wrote. "Along the Mall from the direction of the Palace the sound of an immense concourse singing 'God Save The King' floated in.

"On this deep wave there broke the chimes of Big Ben; and, as the first stroke of the hour boomed out, a rustle of movement swept across the room. The war telegram, 'Commence hostilities against Germany', was flashed to the ships and establishments... all over the world."

The ensuing conflict led to the death of 10 million troops and millions of civilians, completely reshaping Europe's political map.

Newspapers on Monday paid tribute to the World War I dead with special centenary editions.

The Times carried a front-page photograph of a poppy field, while an editorial drew uneasy parallels between today's geopolitical climate and that which led to World War I, supposedly "the war to end all wars".

After mentioning the bloodshed in Gaza and Ukraine, the paper said: "As in 1914, there is a risk of complacency as crises swirl around us."

In a service in Belfast, the Anglican Bishop of Armagh also made the link in an event to mark the centenary in which he described war as an "abject failure of the human spirit and of humanity".

"Without being guilty of the worst kind of religious escapism, we cannot spiritually separate the violence, the carnage and the suffering of the innocent that is under our gaze today - whether in Gaza, in Israel, in Syria, in Ukraine or in Iraq - from our memorialising of the beginnings of the First World War," Richard Clarke said.

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