NORTH DAKOTA (REUTERS) - Dozens of armed law enforcement officials swept through a protest camp near the site of the Dakota Access pipeline Thursday (Feb 23), clearing the camp that for months served as a base of opposition to the multi-billion dollar project.

About 50 police in riot gear, aided by National Guardsmen, moved slowly through the camp in Cannon Ball, North Dakota, checking structures for any of the several dozen protesters who had stayed beyond the Wednesday deadline to evacuate.

The Oceti Sakowin Camp was completely vacated shortly after 2pm (12am Singapore time), the Morton County Sheriff's Department said in a statement.

Forty-seven people were arrested on Thursday, said Ms Maxine Herr, spokesman for the department.

About a dozen Humvee vehicles entered the camp at around 12.30pm and a helicopter flew low overhead, with more police officers and National Guard members stationed outside the site.

Around 200 law enforcement officials from North Dakota, Indiana, Wisconsin and Alabama took part in the operation, Ms Herr said. "Things went very smoothly. We are very happy with the operations."

Members of the Army Corps of Engineers, which owns the land where the camp was built, also began entering the site for cleanup efforts.

By Thursday afternoon, heavy equipment was being used to tear down structures at the site.

"It's really hard being here right now but we know we need to be here because we know that they are wrong and we are right," said Mr Raymond Kingfisher, 59, a protester from Seattle, Washington.

Some protesters were helping with cleanup efforts.

Several buildings that were set on fire on Wednesday ahead of the deadline were still smouldering, sending acrid smoke across the camp.

Other protesters who stayed past the deadline said they were not afraid of another confrontation with law enforcement, which has clashed multiple times with demonstrators, resulting in more than 700 arrests.