Britain's most famous landmarks including the London Eye, Houses of Parliament, Downing Street and Gateshead's Millennium Bridge were plunged into darkness for an hour last night to mark the moment Britain first declared war on Germany in 1914.
The Duchess of Cornwall joined senior politicians tonight for a service of solemn commemoration for those who died in WWI at Westminster Abbey.
Key figures present included Deputy Prime Minister Nick Clegg, former foreign secretary William Hague, Labour leader Ed Miliband and Metropolitan Police commander Sir Bernard Hogan-Howe.
The Duchess lit a candle during the service, as did the 1,700 others in the congregation.
The service also included the gradual extinguishing of candles, with an oil lamp snuffed out at the grave of the unknown warrior at 11pm - the exact hour war was declared.
In the same hour, the nation had been urged to switch off lights in places of worship, public buildings, workplaces and homes, and leave one light burning as a symbol of hope in darkness.
As the Lights Out event to remember those who fought and died in WWI happened last night ITV asked our viewers what they did to take part.
Lights on some of Britain's most famous landmarks were extinguished between 10pm and 11pm to remember those who died in WWI.