Iconic landmarks in cities around the world were plunged into darkness on Saturday night as millions marked Earth Hour.

The annual campaign by environmental NGO WWF is held to demand urgent action to address the climate crisis and to raise awareness of global environmental issues.

Cities go dark in symbolic show of support

In cities across Asia and Europe, a host of buildings switched off their lights in a symbolic demonstration of solidarity for the planet.

The Sky Tower and Harbour Bridge in Auckland, New Zealand were the first to go dark at 8:30 p.m. local time (7:30 a.m. GMT).

Famous landmarks further east in Asia and Europe then joined in as the clock struck 8:30 p.m., including the Sydney Opera House, the Gardens by the Bay park in Singapore, Wat Arun temple in the Thai capital Bangkok, Berlin's Brandenburg Gate, the Colosseum in Rome and the London Eye in the UK.

"Every light turned off is a step towards a sustainable future," Bangkok Governor Chadchart Sittipunt said.

People who had gathered at the Brandenburg Gate in the German capital to mark the event sang songs such as "What a Wonderful World" by Louis Armsrong or John Lennon's "Imagine."

What is Earth Hour?

Earth Hour was launched in Australia in 2007, and has since grown into a worldwide annual event that sees entire city skylines and streets go dark to raise awareness of the climate crisis.

The campaign invites people to "give an hour for Earth, spending 60 minutes doing something — anything — positive for our planet," including planting trees and reducing energy consumption.

In a statement ahead of the 19th edition of the event, WWF International Director General Kirsten Schuijt said the campaign is a "powerful reminder of the urgency to act while we still can."

"The stakes have never been higher. The past year was the hottest on record—capping off the hottest decade, with the hottest seas ever recorded," she said.

"We've witnessed historic wildfires, storms, and droughts as well. Our world is in peril and we are rapidly approaching dangerous climate tipping points beyond which key ecosystems may never be able to recover," Schuijt added. 

Edited by: Wesley Dockery