About 80 people interrupted construction at the North Westover site in rural Hamilton but allowed workers to close and lock the facility before leaving. They are also allowing workers in to do safety checks.
"We're here because we've noticed they've ramped up their construction," said protester Elysia Petrone. "They are making improvements to the north pumping station, preparing to increase capacity."
Petrone added: "This is a complete disregard for the national energy process. This NEB (National Energy Board) process is a rubber stamp procedure."
One hundred and seventy-seven citizens, organizations and municipalities have applied to take part in hearings about Enbridge's plan to reverse the oil flow of Line 9B, which runs from Montreal to Westover near Hamilton. The cities of Hamilton, Burlington, Kingston, Toronto, Mississauga have applied for intervener status, along with musician Sarah Harmer.
'This is my food source'
Petrone says she's concerned about the environmental impact of the Enbridge project.
"This is my drinking water," she said. "I get my vegetables from Plan B (organic farm) about a kilometre from here. This is my food source. To not have a careful environmental assessment, any assessment at all, is shameful."
Protesters have started putting up tents and say they plan to stay for several days. Hamilton police officers arrived at the pumping station at about 9:20 Thursday morning. Officers have told the CBC they hope the protesters will leave peacefully.
Ken Hall, a senior advisor for community relations for Enbridge said "We are willing to speak with these individuals about their concerns. I'm sure we can try to resolve this in due course"
Opponents say the flow reversal could raise the risk of a spill into places like the Beverly Swamp in the headwaters of Spencer Creek, Hamilton's largest watershed.
Earlier this month, representatives from local environmental group known as Hamilton 350 protested outside police headquarters, denouncing $44,410 in grants from the oil giant. Back in May, about 40 protesters staged a mock oil spill and cleanup on Highway 6 near Concession Road 6 to express their concerns about the pipeline.
Hamilton 350 said, in a press release, that protesters at the Westover terminal "look to be in it for the long haul" and have asked supporters to attend a rally on Friday at 11am across the street from the protest.
Calgary-based Enbridge describes itself as operator of the world's longest crude oil and liquids transportation system. It is proposing to build and run the $6.5 billion Northern Gateway pipeline, carrying oil sands bitumen to British Columbia for shipment overseas.
CBC reporter Adam Carter is on the scene in Hamilton. You can follow his tweets below.