Three Canadian activists were arrested early Monday after manually stopping the flow of tar sands oil in Enbridge's controversial Line 9, protesting the pipeline as an affront to Indigenous sovereignty, as well as the planet.

The individuals reportedly bike-locked themselves to a valve site just outside of Sarnia in southwestern Ontario after using the hand wheel to stem the flow.  Police took the three activists, identified as members of the group Rising Tide North America, into custody after removing the bike locks.

Tar sands oil began flowing through the Line 9 pipeline earlier this month after Canada's National Energy Board (NEB) approved the pipeline's reversal, allowing it to carry tar sands diluted bitumen ("dilbit") and fracked Bakken crude from Sarnia, Ontario to Montreal.

The approval came despite the fact that Indigenous groups are in the midst of a Supreme Court challenge against the pipeline, arguing that the government failed to consult with the land holders.

"It’s clear that tar sands projects represent an ongoing cultural and environmental genocide," said Vanessa Gray, an activist from Aamjiwnaang First Nation, who was among those arrested. "I defend the land and water because it is sacred. I have the right to defend against anything that threatens my traditions and culture."

The action is the latest step in a longstanding face-off between pipeline opponents and the Canadian government.

Last week, more than 80 Indigenous and environmental groups sent a letter to Prime Minister Justin Trudeau asking to halt the flow of oil through Line 9 until a proper review can be performed.

"Line 9 poses significant risks to the health and wellbeing of millions of Canadians," the letter stated. "Given your renewed commitments to protecting the environment including the mitigation of runaway climate change as well as your commitments to respecting Indigenous rights, we are calling upon the federal government to restructure the NEB review process. We urge the federal government to halt the Line 9 project until it can be subjected to a subsequent review under more robust, transparent, and democratic conditions."