Hundreds of protesters opposed to a North Dakota oil pipeline project they say threatens water resources and sacred tribal lands confronted police on Sunday at a bridge where a previous clash had occurred, the local sheriff’s department said.

An estimated 400 protesters mounted the bridge and attempted to force their way past police in what the Morton County Sheriff’s Department described as an “ongoing riot.”

A media statement from the agency said one arrest had been made as by 8:30 p.m. local time, about 2 1/2 hours after the incident began.

(Reporting by Steve Gorman in Los Angeles; Editing by Paul Tait)