Lights across Britain will be switched off for an hour on Monday in a tribute to the dead of World War One inspired by the prophetic observation of then-foreign minister Lord Grey on the eve of war 100 years ago. "The lamps are going out all over Europe," Grey said in 1914. "We shall not see them lit again in our lifetime."
British landmarks like Trafalgar Square will be dark from 10pm and Prime Minister David Cameron has asked all 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. One million of the dead were soldiers from Britain and its then empire.
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 as 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.
Politicians and royalty from 83 countries, including presidents Francois Hollande of France and Joachim Gauck of Germany, were among those on hand in Liege while 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 theme of Grey's prophecy will also be at the centre of a service in London's Westminster Abbey later on Monday as the building's lights go out one by one until only a burning oil lamp remains at the Grave of the Unknown Warrior.
At 10pm (2100 GMT), the lamp will be extinguished, marking the exact time the British Empire joined the war. In Trafalgar Square, one single light will shine from an old police box.
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 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 Belgium's Flanders region during the 1914-1918 war.