HOUSTON -- Protest action against the construction of the Keystone XL pipeline is heating up, pitting activists against local law enforcement agencies as their tactics evolve.

Yesterday eight protesters involved in the Tar Sands Blockade movement climbed 80 feet to camp out in treehouses near Winnsboro, Texas, built in an easement given to TransCanada Corp. for construction of the southern leg of the line. The line would connect Houston-area refineries to crude oil storage tanks in Cushing, Okla.

This act of nonviolent civil disobedience is the latest in a handful of incidents that have already landed members of the group in jail. Nevertheless, organizers vow to continue such actions until construction of the line ends.

A TransCanada representative confirmed the protest was taking place. David Dodson added that the company was in the midst of clearing trees to make way for the pipeline and that work crews were nearing the tree encampment.

"We are clearing toward that site," he said. "Actually, we are going both ways toward it."

The southern extension of Keystone XL would initially carry mainly domestic crude oil that's flooding out of the Bakken Shale and other production zones, creating a bottleneck of supply at Cushing. Opponents of the pipeline note that it will eventually carry crude derived from Alberta's oil sands projects.

Many environmentalists oppose oil sands exploitation in general for the footprint it leaves on the landscape and perceptions that it exacerbates the climate change problem. But activists also argue that the heavier-grade bitumen-derived oil carries with it a greater spill risk from a pipeline rupture, and that cleaning it up from the environment is more difficult.

TransCanada insists that the oil the line will carry isn't fundamentally different from standard crude oil and poses no increased risk of spills.

The protesting tree campers have "pledged not to come down until the pipeline is stopped for good," Tar Sands Blockade said in a news release. "Construction cannot proceed until tree-sitters descend and TransCanada clear-cuts through hundreds of trees to make way for the toxic tar sands pipeline."

Last week five people from the same group were arrested and detained overnight after chaining themselves to some equipment being employed by TransCanada crews and contractors in the same area.

Ramsey Sprague, a spokesman for the protesters, said they were all charged with obstruction of a state highway or passageway. They face a hearing for a restraining order with a Franklin County Court judge this week.

"I was actually amongst those arrested at Wednesday's action, despite that I was standing across the street off the road per the instructions of the sheriff," Sprague said. "We were held for over 48 hours, one of us held in a holding cell for 11 hours. It was an awful experience."

Sprague said he expects a confrontation between law enforcement and the eight protesters now occupying trees on the easement to be "imminent." But his group vows to continue fighting the construction of the pipeline.

Dodson said his company is in full contact with local law enforcement authorities regarding the protest.

"The sheriff is aware of our situation," he said. "We appreciate the sheriff's restraint and professionalism in dealing with these protests."

Decisions to enlist the aid of law enforcement in individual cases are being made by crews at the location, Dodson added.