Some of Liverpool's most iconic landmarks will be plunged into darkness to raise awareness of climate change.
People can get a free energy saving light bulb from the council
The Town Hall, St George's Hall, the Cunard building the and world famous Liver Buildings are among 30 places involved in "Lights Out Liverpool".
The campaign, led by Liverpool City Council, is happening from 1900 to 2000 GMT, to highlight global warming.
Merseyside residents are also being urged to turn off their lights and non- essential appliances for the hour.
Lord Mayor, Councillor Paul Clark, said: "Lights out Liverpool is all about sending out a powerful message about the amount of energy we needlessly waste.
"It is really simple if residents continue to work together in reducing energy waste, it really will make a huge difference.
It means Liverpool's famous waterfront will become pitch black.
The lights out campaign has already taken place in London and Sydney this year, when the House of Parliament and Sydney Opera House were in complete darkness.
Councillor Berni Turner, the council's executive member for the environment, said: "Liverpool is aiming to be one of the greenest cities in the UK."
Suggestions to reduce energy, include turning off lights when leaving the room, making sure TVs are not left on standby and switching off photocopiers and printers when leaving work.