A sea of people gathered in downtown Montreal for the annual Earth Day rally, under heavy police presence.

The march was so massive that, more than two hours after it began, a large crowd was still waiting to begin at the starting point.

Many of the demonstrators said they were upset by the Harper government's environmental policies, including its decision to withdraw Canada from the Kyoto Protocol. Others took aim at Quebec Premier Jean Charest's plan to develop the province's north.

"It feels like we're not on the same page," said Melanie Demers, 38, who brought her family to the march.

"It feels as if they're running a business, but I think that it's more than running a business to run a country or a province."

'We are creating the most important ecological, economic and social debt in history, that future generations will have to bear."—Opposition Leader Tom Mulcair

Police were out on horseback, bicycles and foot to oversee security, in light of recent protests in the city that ended in violence.

Earth Day organizers say they contacted Quebec student federations to remind them that Sunday's rally was peaceful and family-oriented.

The reminder came in the wake of a raucous week in the province's ongoing student movement against tuition hikes, in which hundreds of people were arrested during protests held in different cities.

But the focus Sunday was on environmental issues, highlighted every year during Earth Day events around the world.

Opposition Leader Tom Mulcair joined the march, remarking on the environmental "debt" the modern world has accumulated in the last generation.

Police officers on horseback lead Earth Day marchers up Park Avenue. (Willy Lowry/CBC)

"We are creating the most important ecological, economic and social debt in history that future generations will have to bear," Mulcair said in French, as he signed a symbolic petition supporting the Kyoto Protocol.

"We're asking future generations to clean the air, soil and water around tar sands, while we profit."

Parti Québécois Leader Pauline Marois stressed the growing importance of public transit in urban settings.

"We believe that we can further reduce our impact on greenhouse gas emissions by favoring electric transit, and transportation that uses renewable resources," she said.

The march started at 2 p.m. at the Place des Festival, with church bells ringing across the city to mark the occasion. Throngs of people inched northward to Mount Royal Park, waving placards and flags.

Montrealers heeded the city's request for people to use public transit, jamming the underground metro system and buses.

Drivers were being advised to avoid the following areas:

• St-Urbain and Sherbrooke Street.
• Saint-Laurent Boulevard and Pine Avenue.

MAP: 2012 EARTH DAY MARCH IN MONTREAL

With files from Canadian Press