People in 134 countries and territories including the UK switched off their lights for an hour to support action to create a sustainable future for the planet.
Environment charity WWF organised the Earth Hour event which saw buildings such as Big Ben, the BT Tower, Buckingham Palace, Windsor Castle, Edinburgh Castle, Clifton Suspension Bridge in Bristol, Stormont in Northern Ireland and Cardiff's Millennium Centre blacking out for an hour yesterday.
The event, which took place at 8.30pm local time around the world, started in Fiji, New Zealand and Australia.
The aim was to highlight the charity's call on governments, organisations and individuals to pledge their commitment to tackling climate change.
At the Royal Albert Hall in London, television presenter Kirsty Gallacher led a team of 60 cyclists from WWF-UK who created a huge human-powered projection with images of endangered species including dolphins and tigers being shone on to the building.
Colin Butfield, head of campaigns, WWF-UK, said: "Our event at the Royal Albert Hall, alongside the hundreds of thousands of events across the world, shows global support for the need to tackle climate change and protect the natural world.
"The challenge for our future well-being could not be greater. WWF's Earth Hour is about creating a message so powerful that governments and businesses cannot fail to take notice."
Earth Hour started in 2007 in Sydney, Australia, when 2.2 million individuals and more than 2,000 businesses turned their lights off for one hour to take a stand against climate change.
As well as people in 4,000 cities taking part this year, other landmarks which stood in darkness for the hour were Old Trafford, the London Eye, the Eiffel Tower, the Empire State Building, Granada's Alhambra in Spain, Rio de Janeiro's Christ the Redeemer in Brazil, Athens' Acropolis in Greece, India Gate in New Delhi, Sheikh Zayed Grand Mosque in Abu Dhabi and Sydney's Opera House.
Four of the world's five tallest buildings turned off their lights with the tallest, the 828-metre Burj Khalifa in Dubai, switching off about half a million lights.