A group of Mohawks from Kahnawake is preventing freight trains carrying oil or other dangerous materials from passing through the territory on a Canadian Pacific Railway line for 24 hours.
The protest is in solidarity with Indigenous groups protesting against the Dakota Access Pipeline project in North Dakota.
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The blockade began Thursday at midnight. Commuter trains that use the rail line are being allowed to pass.
The AMT is warning commuters who use its Candiac line that the protest may cause delays.
Its trains are authorized to travel on the tracks but will have to do so at reduced speed near the Saint-Laurent train bridge where the protest is taking place.
The bridge connects Montreal and the South Shore.
Jeremy Berry, director of media relations for CP Rail, emailed a statement to CBC.
"While CP recognizes the right to peacefully assemble, this right does not extend to interfering with the nation's freight rail system," the statement read.
Berry refused to say how many trains were affected, or to answer any other questions about how the blockade affected service.
Protecting the earth a 'duty,' protesters say
A spokeswoman for the group, Kahionwinehshon Phillips, read a statement Thursday morning to a group of reporters gathered at a small encampment the group has been maintaining at the foot of the Mercier Bridge for the past few weeks.
"We as the Mohawk people have a duty to protect mother earth, and we will continue to defend our mother earth for the coming generations as our ancestors did," Phillips said.
It's not clear how many trains will be affected. Phillips thought it could be more than a dozen, but she wasn't sure.
Kahnawake Mohawk Peacekeepers are on site to ensure everyone's safety.
Two weeks ago, AMT train service was also suspended for a day due to the Mohawk protest.