The protesters set up barricades made of steel drums and wooden skids at entrances to the Enbridge worksite on Pineway Boulevard, near Finch Avenue East and Leslie Street.
The protesters arrived early, prompting the contractor to send home about 40 workers there to perform routine maintenance on the pipeline. Five of the protesters chained themselves to equipment.
The protesters are opposed to Enbridge's proposal to reverse the flow of oil in the pipeline, which runs from Sarnia, Ont., to Montreal.
The move will boost the line's capacity by 300,000 barrels of crude oil per day, up from the current 240,000 barrels.
Enbridge has said what will flow through the line will not be a raw product from Alberta's oilsands, although there will be a mix of light crude and processed oilsands bitumen.
Opponents say the proposal puts First Nations communities at risk, threatens water supplies and could endanger vulnerable species in ecologically sensitive areas. Enbridge insists the pipeline is safe.
Toronto police admit the protesters are trespassing. A team of officers was on site and speaking with the protesters on Tuesday but as of 12 noon, there's been no attempt to remove them.