London's iconic Houses of Parliament and Big Ben switched off their lights on Saturday (March 28) as part of Earth Hour 2009.
Famous buildings in cities across the world fell dark as part of a World Wildlife Fund (WWF) initiative to encourage people to cut energy use and curb greenhouse gas emissions from fossil fuels.
Organisers said the action showed millions of people wanted governments to work out a strong new U.N. deal to fight global warming by the end of 2009, even though the global economic crisis has raised worries about the costs.
The U.N. Climate Panel says rich nations must cut their emissions by up to 40 per cent of their 1990 levels by 2020 to avoid the worst effects of warming.
Developing nations will also have to slow the rise of their emissions by 2020, it says.