Cities around the world including Dubai and Abu Dhabi turned off their lights on Saturday for Earth Hour, with this year's event highlighting the link between the destruction of nature and increasing outbreaks of diseases such as Covid-19.
In London, the Houses of Parliament, London Eye, Shard skyscraper and Piccadilly Circus were among the landmarks flicking the switches.
"It's fantastic news that parliament once again is taking part in Earth Hour, joining landmarks across the country and the world to raise awareness of climate change," said Lindsay Hoyle, Speaker of the House of Commons.
"It shows our commitment to improving sustainability... and that we're playing our part in reducing energy consumption," he said.
In Paris, the three stages of the Eiffel Tower progressively went dark but there were few people to watch with the whole country under a 7pm Covid-19 curfew.
The giant metal tower has been shut to the public since October 30 because of the pandemic.
In Rome, the lights went out at Rome's 2,000-year-old Colosseum, while police enforcing Italy's coronavirus movement restrictions checked the papers of a small crowd of onlookers.
Asia had kicked off the event after night fell with the skylines of metropolises from Singapore to Hong Kong going dark, as did landmarks including Sydney Opera House.
The Brandenburg Gate in Berlin and Moscow's Kremlin on Red Square also joined the annual initiative that calls for action on climate change and the environment.
After Europe, Earth Hour moved west to the Americas with the Empire State Building in New York, the Obelisk of Buenos Aires and the Rio's Museum of Tomorrow among venues dimming the lights.
