Hundreds of armed state and local police and National Guard — some on foot and others driving trucks, military Humvees and buses — began the operation at midday and slowly enveloped the camp, arresting more than a dozen protesters who refused to leave.

There were no serious injuries, although one man was hurt in the leg and received treatment from a medic.

Protesters initially set up roadblocks and started some fires to slow the law enforcement advance but eventually retreated.

Morton County Sheriff Kyle Kirchmeier said that the camp had been cleared by nightfall although police were still dealing with protesters on the perimeter, and he said police would stay put for now.

''We're not leaving the area,'' Kirchmeier said. ''We are just going to make sure that we maintain a presence in the area so the roadway stays open, and to keep individuals from camping on private land.''