LONDON, ON - Protesters have brought to a halt a hearing by the National Energy Board in this southwestern Ontario city that environmentalists fear will bring oilsands oil to Ontario and eventually the Unites States.

Minutes after the hearing began, protesters in the hearing room at the Hilton London began screaming chants.

Board members and Enbridge quickly cleared the room and natives from the Six Nations replaced them at the front of the room to speak out and detail their grievances.

London police say they are planning how to re-gain control so the hearing can continue.

Enbridge wants to reverse the flow of oil in an Ontario pipeline so it moves from west to east. Whether that's a boon to Ontario's economy or a threat to its environment is a matter of fierce debate at the hearing that began Wednesday and was expected to last three days.

Environmentalists point to a rupture two years ago of an Enbridge pipeline in Michigan.

Oil from the oilsands is heavier and more corrosive than lighter crude, environmentalists say, a claim disputed by Enbridge. As for the pipeline break in Michigan, investigators haven't determined a cause, the company says.

Environmentalists also argue Enbridge's push to reverse the flow of oil is just the first step toward moving oil to the US east coast, something they fear would speed extraction and oilsands production and further degrade the global environment.

In a conference call this month with media and analysts for the benefit of investors, Enbridge's leaders said it might someday renew efforts to get oil to the east coast. Environmentalists have asked the energy board to consider the broader effect of moving oil across Canada and in the US, but the board has repeatedly refused to do so.

That's frustrated environmentalists convinced Prime Minister Stephen Harper's Conservatives want to make Canada a petrostate - more than 40,000 have written the energy board, many from south of the border.

But Enbridge says current market conditions don't support moving the oil to the US east coast.

Enbridge last week said it would also apply to reverse oil flow as far east as Montreal. The energy board won't give a thumbs up or down on Enbridge's proposal but will decide on the breadth of the environmental review, said Albert Koehl, a lawyer with Ecojustice representing environmental groups at next week's hearing.