Hundreds of police officers in riot gear moved in late on Thursday to force activists off private property close to the site of a controversial oil pipeline in the northern US state, local officials said.

Tensions rose in the evening when gunfire was heard at the site, Molotov cocktails were thrown at officers and fires were started, according to the officials.

Most of the 141 protesters detained were held for conspiracy to endanger by fire or explosion, engaging in a riot and maintaining a public nuisance, said Donnell Hushka, a spokeswoman for the Morton County Sherriff's Department.

The operation to evacuate the area began a day after they refused to leave voluntarily. No serious injuries were reported, though one man was hurt in the leg and received treatment from a medic.

Native Americans angry

Plans for the 1,931-kilometer (1,200 mile) oil pipeline, which begins in North Dakota and is to cut through the states of South Dakota and Iowa before ending in Illinois, have led to widespread hostility from environmental activists and Native Americans.

A spokeswoman for the Sioux tribe told the Agence France-Presse news agency that "security forces responded disproportionately" by using military vehicles and water cannons to remove protesters.

The Indian tribe says the pipeline threatens its drinking water sources and several sites where ancestors are buried.

The tribe has challenged a US Army decision granting permits in the courts.  In September, a federal judge denied its request to block construction, but three federal agencies stepped in to order construction to temporarily halt on part of the land around Lake Oahe.

Meanwhile, construction has continued on private land owned by the developer, with a goal of completion by the end of the year.

Camp cleared

Morton County Sheriff Kyle Kirchmeier said that the camp had been cleared by nightfall and police would stay on the site before handing it over to a private security company.

The camp cleared on Thursday is located just to the north of a more permanent encampment, which has been the main staging area for hundreds of protesters.

Cody Hall, a spokesman for the protesters, vowed a new camp would be built elsewhere in the pipeline's path, but on federal land.

"It's going to take a lot to move [the protesters] from there," he said.

mm/kl (AFP, AP)