Thousands of protesters rallied in Washington D.C. Sunday to demand that U.S. President Barack Obama reject the proposed Keystone XL pipeline.

Braving below freezing temperatures, demonstrators carrying banners marched on the National Mall. The rally was organized by environmental groups who denounce the project, which would transport Aliberta oilsands product to refineries along the U.S. Gulf coast.

Rally organizers, who say the project would contribute to global warming, called the rally the biggest climate protest in American history.

Though organizers pegged the crowd at the foot of the Washington Monument at 50,000-strong, one police officer in attendance said he thought the figure was closer to 10,000 as the rally got underway.

Protesters arrived from 28 states for the rally, The Canadian Press reported.

Although demonstrators demanded U.S. government action on fracking and coal, the rally took aim at the Keystone pipeline expansion.

Protesters converged on Washington dressed as polar bears and Lady Liberty, shouting 鈥榥o to Keystone鈥 and referring to the proposed pipeline as a 鈥榗arbon bomb,鈥 CTV Washington correspondent Joy Malbon reported.

Sierra Club executive director Michael Brune was one of several high-profile environmentalists to address the rally.

鈥淧resident Obama holds in his hand a pen and the power to deliver on his promise of hope for our children,鈥 he said. 鈥淭oday, we are asking him to use that pen to reject the Keystone XL tar sands pipeline, and ensure that this dirty, dangerous, export pipeline will never be built.鈥

Ottawa has approved the $7-billion megaproject, so its fate now lies with U.S. Secretary of State John Kerry. Within weeks, the environmental draft assessment of the new route will be released by the State Department, followed by a cooling off period during which citizens will be invited to write in their opinion on the pipeline. Obama will make a final decision on whether to approve the proposed pipeline.

Canada鈥檚 Conservative government has said the country鈥檚 economy, employment and national security stands to benefit from the Keystone project.

Jay Ritchlin, a director general with the David Suzuki Foundation, says Canada could seek energy alternatives to invest in jobs and economy in a way that will reduce carbon emissions.

The foundation recently reviewed low carbon energy scenarios from other industrialized, wealthy economies and it indicates Canada can follow suit while increasing the country鈥檚 Gross Domestic Product each year, Ritchlin told 麻豆影视 Channel Sunday.

鈥淲e really believe that there are possibilities that include both a strong economy and a very strong reduction in greenhouse gases,鈥 he said.

The proposed pipeline -- which would transport Alberta oil more than 3,000 kilometres to refineries in Texas, crossing through multiple U.S. states -- has been protested against by environmental groups both in Canada and the U.S. over concerns that it harms the environment.

Obama is not witnessing the rally firsthand as he is vacationing in Florida for the Presidents鈥 Day long weekend.

In his state of the union speech last week, Obama challenged U.S. congress and Canada to take aggressive action on climate change.

Baird responds to climate change comments

Foreign Affairs Minister John Baird said Sunday the United States could learn some lessons from Canada on reducing greenhouse gas emissions.

In a telephone interview with The Canadian Press, Baird said Canada has adopted the same goals and objectives as the U.S. in terms of climate change, and has harmonized vehicle emission and light truck standards.

鈥淲e鈥檙e also taking concrete direct action with respect to dirty, coal-fired electricity generation,鈥 Baird said. 鈥淢aybe the United States could join Canada on that file.鈥

His comments came days after U.S. Ambassador David Jacobson said Obama鈥檚 call for action from Congress on climate change was also aimed at Canada.

On CTV鈥檚 Question Period Sunday, Saskatchewan Premier Brad Wall said he has sent a letter to Jacobson asking for clarification over comments from the White House that appeared to link approval of the Keystone pipeline project with Canadian environmental policy reforms.

Wall acknowledged that Canada needs to do a better job of getting its environmental message out, but said the country is 鈥渄oing more than having a discussion鈥 about new regulations in the oil industry.

With Files from The Canadian Press