Protesters set up a blockade at an Enbridge pipeline site in southwestern Ontario on Tuesday, disrupting work on what is called Line 9.
The demonstration at Innerkip north of Woodstock started early in the morning and was aimed at preventing installation of a valve.
Protest spokeswoman Rachel Avery said the valve was being installed near the Thames River, which isn't adequately protected from the risk of a spill.
"We're very concerned about the severe impacts of Line 9 on waterways, including the nearby Thames river," Avery said.
Provincial police said there were about 25 protesters involved.
The group has no formal name but Avery said the blockade is dubbed "Dam Line 9!" and has attracted people from "numerous" communities in southwestern Ontario.
Enbridge's (TSX:ENB) Line 9 carries crude oil between Sarnia, Ont., and Montreal. The company plans to reverse the flow of oil and increase the pipe's capacity, but opponents argue the pipeline puts the environment at risk and there has been little consultation with aboriginal communities.
Enbridge confirmed Tuesday's work interruption and said workers were moved to another project for the day.
Spokeswoman Kristen Higgins said the site is "secured" and that Enbridge respects the rights of individuals and groups to voice their concerns.
"I can't speculate on what the future is going to hold for this," Higgins said. "We share concerns about safety and maintaining the safety of the line."
Higgins also said Line 9 has a good safety record.
Earlier this year, Enbridge's plan to flow oil eastward was approved by the National Energy Board, which said its decision would allow the company to "react to market forces and provide benefits to Canadians, while at the same time implementing the project in a safe and environmentally sensitive manner."
Opponents of the Line 9 project often refer to Enbridge's spill in Michigan, where 20,000 barrels of crude leaked into the Kalamazoo River in 2010.
Avery said the protesters planned to stop the work "indefinitely."
"It is a health risk, it is an environmental risk and it doesn't even provide much economic benefit, for very, very grave costs," she said.
The group is also protesting Line 9, Avery said, because it facilitates expansion in Canada's tar sands region.
The federal government has said Enbridge's project will protect high-quality refining jobs in Quebec, open new markets for oil producers in Western Canada and replace more-expensive foreign crude.