Enbridge spokesman Ken Hall said protesters arrived at the company's Line 9 pumping station in North Westover at 6:30 a.m., and forced the workers to shut it down.
Enbridge (TSX:ENB) has already received approval by the National Energy Board to reverse the flow of oil between Sarnia, Ont., and North Westover. It is now waiting for approval to reverse the flow on the North Westover-Montreal segment.
The company also plans to increase the line's capacity from 240,000 barrels per day to 300,000.
Activists say they do not want Alberta's crude oil to flow through their community.
"These pipelines are in a big way the arteries of the tar sands," said David Prychitka, who joined the protest Thursday.
"If we can stop this one here then the tar sands oil will essentially be bottle-necked."
Hall said the company had applied for the possibility to move heavy crude oil.
"We basically asked for a permit which provides flexibility that we could move any type of crude oil that our customers asked us to ship for them."
Protesters said they were unsure how long they would be camped out at the Enbridge site.
"It's impossible for us to say right now because we are unsure how the police and Enbridge are going to respond," said Prychitka.
Hamilton Police spokeswoman Debbie McGreal-Dining said three police officers were on the scene monitoring the situation.