Landmarks around the world went dark, 60 minutes at a time, for the fourth annual Earth Hour.

The Sydney Opera House was one of the first buildings to turn out the lights in an international call for action on climate change.

Environmental organizations expect that close to a billion people living in 3400 cities around the globe will take part.

Here in Montreal, the light on Mount Royal was extinguished, as was the iconic 'Q' on the Hydro Quebec tower, between 8:30 and 9:30 p.m, but while concerts to mark the event took place in Toronto and Halifax, no mass gatherings were planned for this city.

Hockey fans did not participate, since the Canadiens were playing host to the New Jersey Devils at the Bell Centre.

Earth Hour was first started by the WWF in Sydney, Australia in 2007 as a way to reduce power use and send a message about the dangers of climate change.

Since that first year, the one-hour, voluntary blackout has spread like wildfire.

In Canada, the event attracts a number of participants that is unmatched by nearly any other occasion, according to Keith Stewart, WWF Canada's climate change director.

"In terms of participation, the only time you get more Canadians doing the same thing at the same time is watching the gold medal hockey game," Stewart told CTV News Channel Saturday. "We had over 10 million Canadians participate last year. The skylines do go significantly darker."

With files from News Staff