Environmental activists continue their sit-in at the White House for a fourth day today, despite dozens of arrests.

They are calling on U.S. President Barack Obama to reject the permits required to develop Keystone XL, a 3,200-kilometre pipeline that would carry crude from Alberta's oil sands to Texas refineries.

The Tyee's Geoff Dembicki has reported extensively on the political war that has been raging in Washington over this decision.

While Canadian officials have been aggressively selling U.S. Congress on the oil sands and proposed pipeline, a coalition of environmental groups from both sides of the border has managed to stall U.S. approval of the project for months.

Author, activist and scholar Bill McKibben is leading the demonstration. He was among those arrested Saturday, and was released yesterday afternoon without charges.

"This is the single key environmental test for Barack Obama between now and the election," McKibben told the Burlington Free Press (a paper based in McKibben's home state of Vermont) following his release. "If he does the right thing, it will send a jolt of electricity to the millions who went out and worked for the guy in 2008."

According to the Environmental News Service, 162 people have been arrested so far. The sit-in is taking place in the so-called "postcard zone" in front of the White House, which is not allowed by the U.S. Park Service. According to reports, new people are arriving every day to take the place of those detained and approximately 2,000 have signed up to take part in the sit-in, which is supposed to continue until September 3.

Colleen Kimmett reports for The Tyee.