British activists take part in a mobile phone flashmob at Parliament Square in London on Monday. (Matt Dunham/Associated Press)

Activists across Canada and around the world used their mobile phones on Monday to stage demonstrations urging world leaders to act on climate change.

The protests were planned as a "wake-up call" for world leaders in advance of the United Nations summit on climate change in New York on Tuesday.

Participants in so-called "flash mobs" held up mobile phones synchronized to sound alarms at the same local time, 12:18 p.m., a time chosen because it corresponds with Dec. 18 — the 18th day of the 12th month — and the last day of expected climate treaty talks in Copenhagen, Denmark.

Flash mobs are planned brief events — part performance art — that use mobile phones to appear to mobilize people spontaneously and quickly to the amazement of onlookers.

Activist group Avaaz, which organized the protests, said 189 events had been planned across Canada.

Canadian demonstrators used their phones to leave messages with politicians during a two-minute window. Protests were scheduled in Halifax, Montreal, Ottawa, Toronto, Calgary, Edmonton, Whitehorse, Yellowknife and Vancouver and many smaller Canadian communities.

Julia Morgan, who organized an event that brought 30 people to the intersection of King and Church streets in downtown Toronto, said calls across the country flooded the inboxes of Canadian politicians.

However, Morgan said, she was only able to get through to federal Liberal Leader Michael Ignatieff's secretary.

"She said she'd already received several calls and I registered my opinions on the very strong need for the Canadian government to make a fair, binding and ambitious deal in Copenhagen in December," Morgan said

Court Ellingson, who attended the Toronto event, said he had left a message with an administrator of Alberta MP Chris Warkentin in his riding of Peace River.

In northern Canada, noon-hour rallies attracted several dozen people each to downtown Whitehorse and Yellowknife.

Some demonstrators said they are concerned about the effects of climate change on wildlife, which northern indigenous peoples rely on for food.

"Our caribou's on a drastic decline, our salmon is on a drastic decline, all our foods are on a drastic decline," Norma Kassi, a member of the Vuntut Gwitchin First Nation in Old Crow, Yukon, said during the Whitehorse rally.

"We're going to have a real issue of food insecurity and health pertaining to our people, especially indignous peoples who live on the land."

UN Secretary General Ban Ki-Moon invited world leaders to New York on Tuesday in a bid to mobilize the "political will" needed to seal a deal at the UN Climate Change Conference in Copenhagen, which begins Dec. 7.

With files from the Canadian Press