TransCanada is building the domestic leg of the project in the United States while it awaits federal approval to move on the artery slated from Canada.
Project supporters say the project is vital for U.S. energy security and would boost the economy. Critics said Canadian crude oil is more harmful to the environment and its corrosive nature makes pipelines more susceptible to spills.
Advocacy group Tars Sands Blockade said protesters chained themselves to heavy machinery and blocked a construction area.
Ron Seifert, a spokesman for the group, said acts of civil disobedience were meant to protest "TransCanada's fraud, bullying, and reckless endangerment of their lives and fresh water."
Advocacy group Oil Change International reported that a bipartisan group of 18 U.S. senators who called on U.S. President Barack Obama to sign off on the project "has taken on average, roughly twice as much money over their careers from fossil fuel interests as has the rest of the Senate."
TransCanada has said safety is a top priority for Keystone XL. Marty Durbin, executive vice president for the American Petroleum Institute, said last week that "if we can source more of that energy from right here in North America while creating jobs and generating government revenue, approving the Keystone XL pipeline should be an easy decision."