Summary Cameron began a day of remembrance by attending a service at Glasgow Cathedral.
LONDON (AFP) - Lights will go out in homes across Britain on Monday as part of commemorations marking 100 years since the outbreak of World War I, which claimed the lives of nearly one million Britons.
Millions of households were encouraged to turn off their lights from 10 pm (2100 GMT) to 11 pm and to light 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 single candle will be 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.
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 began a day of remembrance by attending a service at Glasgow Cathedral, following on from the Commonwealth Games in the city.
"It is right to remember the extraordinary sacrifice of a generation, and we are all indebted to them because their most enduring legacy is our liberty," Cameron told the BBC.
More than 1,100 people from around the Commonwealth attended the service, which was attended by Britain s political elite and Peter Cosgrove, governor-general of Australia, and Gordon Campbell, high commissioner for Canada.
Reverend Laurence Whitley, minister of Glasgow Cathedral, said: "We meet because on a summer s day like this one, 100 years ago, the world changed. Our nations and peoples found themselves in a war the like of which had never before been seen and the memory of which still haunts us all."
Queen Elizabeth II attended a commemoration at Crathie Kirk, the small Scottish church used by the royal family when holidaying at nearby Balmoral Castle.
The queen was represented at the Glasgow service by her son and heir-to-the-throne Prince Charles, who later laid a wreath at the city s Cenotaph with Cameron.
Charles s wife Camilla was later to attend a service at London s Westminster Abbey, while Prince William and his wife Catherine marked the occasion at commemorative events with European leaders in Belgium.
Prince Harry unveiled a memorial arch in Folkestone, south England, before laying a wreath at the town s war memorial.
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 paid tribute to the dead with special centenary editions on Monday.
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."
