Screengrab via YouTube.

Late on Sunday, protesters at the encampment near the North Dakota Standing Rock Sioux reservation were corralled on a bridge and assaulted by police with water cannons, tear gas, pepper spray and rubber bullets. The police are characterizing the incident as a “riot.”

CNN reports that protesters attempted to cross Backwater Bridge, north of the camp site, in an attempt to walk on Highway 1806. RT reports that some protesters were using a semi truck in an attempt to remove burnt-out military vehicles near the bridge that were left over from a protest earlier in November, and that’s what sparked the confrontation.

Police are also saying that several fires were started on the bridge, and that their use of water cannons on a night that fell to 28 degrees Fahrenheit was necessary to control both the protesters and the flames. Video captured by drones does show some seemingly contained fires, but at a distance from where protesters are being sprayed:

Only one person was arrested out of the reported 400 on the bridge, but a number of people have supposedly been shot in the face and head by rubber bullets.

Senator Bernie Sanders tweeted a livefeed of the protest at the President, saying he should protect the protesters: