Hundreds of environmentalists and other activists in Canada made their websites go dark Monday to protest planned measures to facilitate oil pipeline construction.

The groups accuse the Conservative government of Prime Minister Stephen Harper of trying to silence opponents of exploitation of Alberta's tar sands, the third-largest oil reserves in the world, through new environmental legislation it is pushing through Parliament.

The measure, included in a budget bill, allows the federal government to have the final word on approving major pipelines, as well as override National Energy Board decisions, speed up reviews of resource development projects and keep citizen groups out of environmental reviews for pipelines.

It also allocates $8 million to the Canada Revenue Agency to fund audits of charities, which activists labeled an effort to silence advocacy groups speaking out on important environmental issues.

"Under the proposed bill, Ottawa will use its power to promote oil and other major industries instead of taking care of Canada's environment, wildlife and people," read a statement from the groups.

Among the more than 500 organizations activists said were participating in the "Black Out, Speak Out" protest included World Wildlife Fund Canada, Greenpeace Canada and the David Suzuki Foundation, but also the websites of musician Bruce Cockburn and author Margaret Atwood.

US-based groups such as the Natural Resources Defense Council and 350.org also participated by darkening their websites.

"The darkening of websites and the thousands of letters, blogs, tweets and other actions by landowners, businesses, First Nations, trade unions, scientists and citizens, reflect the grave concern and deep frustration Canadians feel about the direction the federal government is heading," said Alex Neve, secretary general of Amnesty International Canada.

"Fundamental human rights such as freedom of expression are at stake."

Environmental leaders held a press conference speaking from behind the bars of a wooden prison cell painted in black and placed in downtown Montreal.

"It's the very face of the country that is changing before our eyes," warned Steven Guilbeault, co-founder and deputy executive director of Equiterre.

"Under this new environmental law, fish would only have value if they have economic value. If they don't have economic value, there is no environmental assessment."

John Fraser, a former Conservative fisheries minister said the "far-reaching" bill was putting key fish species at risk.

"It's bad policy and it's bad democracy," he said. "I'm speaking out today because I'm a Conservative and nobody can pretend to be a real Conservative if they are not a conservationist."