The demonstration at Innerkip north of Woodstock that started early today is aimed at preventing installation of a valve.
A protest spokeswoman says the valve is being installed near the Thames River, which isn't adequately protected from the risk of a spill.
Enbridge confirmed the work interruption and says workers were moved to another project for the day.
The company's Line 9 carries oil between Sarnia, Ont., and Montreal.
Enbridge plans to reverse the flow of oil and increase the line's capacity, but opponents argue the pipeline puts the environment at risk and there has been little consultation with aboriginal communities.