More than 7,000 cities have switched off their landmarks for an hour in a joint call for global action on climate change.
The Earth Hour movement aims to create greater awareness and more sparing use of resources, especially fossil fuels that produce carbon gases and lead to global warming.
Beginning in Sydney in 2007, Earth Hour – organised by the World Wildlife Fund – has spread to more than 180 countries, with tens of millions of people joining in.
In London, Buckingham Palace, the Houses of Parliament, the London Eye and the Shard all went dark to show their support.
Across the rest of the UK, iconic landmarks such as Cardiff Castle, Liverpool Cathedral, Old Trafford, Brighton Pier and the Eden Project also turned off in an effort to go green.
In Hong Kong, major buildings along Victoria Harbour turned off their non-essential lights and the citys popular tourist attraction known as the Symphony of Lights was cancelled.
Over 3,000 corporations in Hong Kong signed up for Earth Hour 2019, according to the WWF Hong Kong website.
Skyscrapers including the Bank of China Tower and the HSBC Building in the citys major business district, switched off their lights in response to the global movement.
The City of Lights also turned off the Eiffel Towers nightly twinkle to mark Earth Hour.
Paris Mayor Anne Hidalgo dimmed the lights on the citys most famous monument for an hour.
In Taipei, Taiwans capital, the islands tallest building, Taipei 101, joined surrounding buildings in shutting off their lights.