SARNIA, Ont. -- Enbridge Inc.'s Line 9 pipeline was offline for about 90 minutes on Monday morning after three activists concerned about the line's environmental impacts locked themselves to a valve site on the outskirts of Sarnia, Ont.

The line, which runs from southwestern Ontario to Montreal, is back in service and crude deliveries have not been affected, said company spokesman Graham White.

Lindsay Gray, speaking on behalf of the trio of "land defenders" who staged the protest, said it was easy to get into the fenced-in valve site.

"They had no security whatsoever. Anyone could have done this -- anyone," said Gray, a member of the Aamjiwnaang First Nation, by the starting point of the pipeline

The activists sought to manually shut off the pipeline by turning a big blue wheel once they got into the site. White said the company shut off the flow remotely from Enbridge's control room as a safety precaution shortly after the protest began.

The group phoned Enbridge from the scene and company representatives and police arrived shortly thereafter. Gray said the three were then taken into custody.

A similar demonstration disrupted Line 9 operations in Quebec two weeks ago.

Gray says First Nations along the route were not properly consulted about the project, which she says poses environmental dangers.

Line 9 has been in operation for about four decades. It originally flowed from west to east, but then in the late 1990s, its flow was reversed to bring imported oil inland.

Enbridge (TSX:ENB) recently finished up a controversial project to restore Line 9 to its original configuration and boost its capacity, enabling it to supply Alberta crude to Suncor Energy's (TSX:SU) Montreal refinery.

"The fact that line 9 is currently in operation really just adds to the urgency for people to act," protester Stone Stewart said in a statement. "I'm here because the negative impacts of the oil industry are taking place right now, every day."