A woman is seen sitting inside her vehicle during dusk in the Rosebud camp where "water protectors" continue to demonstrate against plans to pass the Dakota Access pipeline adjacent to the Standing Rock Indian Reservation, near Cannon Ball, North Dakota, U.S., December 7, 2016. Picture taken December 7, 2016. REUTERS/Stephen Yang

(Reuters) - Three protesters were arrested on Monday for trespassing along the Dakota Access Pipeline construction site, police said, where a fierce months-long demonstration has unfolded against the building of a multibillion-dollar pipeline.

The site of the $3.8 billion project in southern North Dakota had been the scene of demonstrations by Native Americans and environmentalists, who say the pipeline would damage sacred lands and any leaks could pollute the water supply of the Standing Rock Sioux tribe.

But in early December the U.S. Army Corps of Engineers denied a key easement needed to allow the pipeline to run under Lake Oahe, a reservoir formed by a dam on the Missouri River.

The protesters, who were taken into custody on Monday for criminal trespassing onto private property, inciting a riot and resisting arrest, were among about 200 people who demonstrated near the fenced-off Dakota Access Pipeline horizontal drill pad, the Morton County Sheriff Department said in a statement.

The three unidentified people were accused of cutting security wire, removing fencing material and dismantled lights that are used to illuminate a bridge near the confluence of the Cannonball River and the Missouri River, the sheriff said.

There were reports on social media that police fired tear gas on protesters on Monday. Officials were not immediately available for comment.

North Dakota Governor Doug Burgum, who took office last month in the height of tensions surrounding the Dakota Access Pipeline, said last week he believed the line would eventually be built and asked opponents to clean their protest camp before spring floodwaters create a potential ecological disaster.

Law enforcement has made a total of 589 arrests during protest activity since August 10, according to the sheriff.