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"The lamps are going out all over Europe," Edward Grey told an acquaintance, shortly before Britain declared war on Germany on Aug. 4, 1914. "We shall not see them lit again in our lifetime."
British landmarks, including the Houses of Parliament, Tower Bridge and St Paul's Cathedral, went dark from 10 p.m., and Prime Minister David Cameron had asked Britons to switch off all but a single light in their homes for an hour.
The "war to end all wars" spread carnage across Europe, especially northern France and Belgium, killing 17 million soldiers and civilians in 1914-18. Over one million of the dead were soldiers from Britain and its then empire.
Grey's prophecy was also at the centre of a service in London's Westminster Abbey later on Monday, where candles went out one by one until only a burning oil lamp remained at the Grave of the Unknown Warrior.
At 11 p.m. (2200 GMT), the lamp was due to be extinguished, marking the exact time the British Empire joined the war. In Trafalgar Square, one single light shone from an old police box.
Acting as beacon for the capital, a monumental pillar of light beamed into the clouds from Victoria Tower Gardens. Installation "spectra" by Japanese artist Ryoji Ikeda was commissioned by 14-18 NOW, the official cultural programme for the centenary, and will fade away as the sun rises over the London skyline on Aug. 11.
"The light that 'spectra' throws up into the night sky is a unifying point; it echoes how the First World War affected all Londoners, but also how they and the rest of the country came together, standing united during those dark days," Boris Johnson, the mayor of London, said in a statement.
FRIENDS AND ALLIES
Prime Minister Cameron and Prince William, second in line to the throne, attended 100th anniversary ceremonies in Scotland and Belgium on Monday. Speaking at an event in Liege, William paid tribute to those who died. He noted that the current fighting in Ukraine showed instability continued to stalk Europe.
"We were enemies more than once in the last century and today we are friends and allies," the prince said, alluding to Germany and its cohorts in the first and second world wars.
"We salute those who died to give us our freedom. We will remember them," he told Belgium's King Philippe and other heads of state attending the Liege ceremony at the Allies' Memorial, near to where German troops invaded Belgium in the early hours of Aug. 4, 1914 - the event which brought Britain into the war.
Commemorations in Germany are understated, with no national initiative to remember the war. But Germans have been encouraged to place flowers on soldiers' graves and many local, small-scale efforts marked the anniversary.
In Munich's city centre, a white hot-air balloon was tethered to the ground as a symbol of hope and peace, and artist Martin Schmidt installed his work "Kraterfeld" - a lawn littered with craters and small bumps, replicating the shell explosions and trenches in landscape around Verdun, northeast France.
Politicians and royalty from 83 countries, including presidents Francois Hollande of France and Joachim Gauck of Germany, were among those in Belgium. In Glasgow, Scotland, Cameron was joined by heir-to-the-throne Prince Charles at a centenary service.
"When you think that almost every family, almost every community was affected, almost a million British people were lost in this war, it is right that even 100 years on, we commemorate it, we think about it and we mark it properly," the Conservative prime minister told the BBC earlier on Monday.
The war's most enduring symbol, poppies, featured at the Tower of London with an art installation called "Blood Swept Lands and Seas of Red" by Paul Cummins, in which thousands of ceramic poppies flow from the medieval monument's wall into the dry moat.
The artwork will grow throughout the summer until 888,246 poppies have been added to represent each British or Colonial fatality during the war - more than double the number of Britain's casualties in World War Two.
Red poppies have become a symbol of remembrance since the trench warfare waged in the poppy fields of the Belgian region of Flanders during the war.