BISMARCK, N.D. -- Law officers arrested about three dozen Dakota Access oil pipeline protesters in a confrontation Friday that also shut down a state highway.

The midday incident began after about 100 protesters confronted crews doing dirt work along the pipeline route where pipe had already been laid. Workers were safety evacuated, but protesters threw rocks, vandalized equipment, slashed tires on Highway Patrol vehicles, and used themselves and vehicles to block a county road and state Highway 6, according to Morton County sheriff's spokeswoman Donnell Hushka. Authorities temporarily shut down a 10-mile stretch of the highway for public safety reasons.

There were no immediate reports of injuries.

The clash happened about 20 miles to the northwest of a protest camp where hundreds of pipeline opponents have gathered for months. More than 470 people have now been arrested since August. Cody Hall, a spokesman for the protest camp, said he couldn't comment on Friday's clash because he wasn't present and didn't have details.

The 1,200-mile pipeline that's to deliver oil from North Dakota through South Dakota and Iowa to a shipping point in Illinois is complete except for under the Missouri River in North Dakota. Work on that stretch has been delayed while the Army Corps of Engineers reviews its permitting. The Standing Rock Sioux and other opponents say the pipeline threatens drinking water and cultural sites. The company insists it's safe.

Energy Transfer Partners said in a statement Friday that it's made an offer to the state to help pay law enforcement costs related to the protests "but it has not moved beyond that at this time." A spokesman for Gov. Jack Dalrymple said the governor's office hadn't seen the offer yet and wouldn't speculate on whether the state would accept.