Some of the UK's most famous landmarks plunged into darkness as millions of people turned off their lights as part of an hour-long international switch-off to raise awareness about climate change.
Buckingham Palace, London's Tower Bridge, Old Trafford football stadium and the Elizabeth Tower, widely known as Big Ben, all took part in the WWF's Earth Hour.
The event, which took place in the UK from 8.30pm, saw more than 700 landmarks around the world switch off their lights, including Christ the Redeemer in Brazil, France's Eiffel Tower and more than 30 Unesco World Heritage Sites.
Within half an hour of the switch-off EarthHour2015 was trending on Twitter with people posting pictures of candles in the darkness.
The Palace of Westminster, Blackpool Tower and Edinburgh Castle were also among other UK sites that took part.
Visitors to London's South Bank - the "focal point" for the Earth Hour switch off - were offered candlelight dinners in many of its bars and restaurants, the WWF said.
Tennis star Andy Murray, a WWF global ambassador, was among a host of famous faces backing the campaign along with Prime Minister David Cameron and Labour leader Ed Miliband, the charity added.
Earth Hour was launched in 2007 in Australia and has since spread around the world.
Last year nine million people in the UK took part as part of a record-breaking chain of 162 countries.