Luminaries including Jeremy Paxman, Twiggy and David Gandy are joining the campaign to turn the lights out across Britain tonight, to mark the 100th anniversary of the country entering the First World War.
The lights of Major landmarks and people's homes will all be switched off and candles lit at 10pm in memory of the devastation and carnage the war wrought.
Millions of people are expected to participate in the Lights Out campaign - where people will switch off all electric light and light a single candle to mark today's centenary - between 10pm and 11pm tonight.
Jeremy Paxman has joined the campaign for #LightsOut
Hundreds of councils, iconic buildings, national organisations, including the BBC and Royal British Legion and places of worship are also expected to take part.
National landmarks buildings including Blackpool illuminations, the Houses of Parliament, Eden Project, the Imperial War Museums and Tower Bridge will also turn off their lights.
London's Piccadilly Circus and the Royal Chelsea Hospital are among the venues that will go dark.
...and model David Gandy
Twitter is awash with people pledging to switch off their lights under the hashtag #lightsout, as well #lestweforget to mark the range of services taking place today, the centenary of Britain entering the war.
Lights Out, organised by veterans' charity The Royal British Legion, is a tribute to the remarks made by Sir Edward Grey, the foreign secretary in 1914, who said on the eve of the war: "The lamps are going out all over Europe and we will not see them lit again in our lifetime."
The charity called it "a symbolic act of reflection and hope".
Kate Middleton is expected to attend a candle-lit service at Westminster Abbey from 10pm, which will extinguish the last candle at 11pm, the moment Britain declared war.
The Royal Chelsea Hospital, whose famous red-coat wearing pensioners are all veterans, will switch off all electric light and have a single candle burning for the hour.
The Legion has produced a video promoting the campaign, which includes figures such as Jeremy Paxman, serving military personnel and Chelsea Pensioners lighting candles.
The Royal British Legion is calling on people to switch off artificial lights to mark the war's centenary
The Legion says it hopes 1 million candles will be lit to "remember each and every one of those Service men and women who gave their lives in the war to end all wars".
But not everyone is thrilled that people are getting in on the act.
In Sussex, firefighters warned those planning to use candles to be careful not to put them next to their curtains or place them in reach of their pets or children.
“We certainly don’t want to discourage anyone from taking part in this national moment of reflection. We just want to remind people to take some simple precautions if they are planning to light a candle," a spokeswoman said.