The scene at the Dakota Access Pipeline protest turned chaotic in North Dakota on Sunday night.
The Morton County Sheriff's Office characterized the events as a riot and estimated 400 protesters were on a bridge trying to go north on state Highway 1806 late Sunday evening. Law enforcement had formed a line to prevent their movement.
Dallas Goldtooth, an environmental activist and organizer for the Indigenous Environmental Network, said in a Facebook post the protesters were met with "water cannon, tear gas and concussion grenades."
Protestors also said firefighters are intentionally spraying them with a water cannon in sub-freezing temperatures.
However, a spokesperson for the fire department says the firefighters are using hoses to put out fires set by protesters.
According to the Associated Press, at least one person had been arrested.
Several Facebook live videos have circulated, but many are blurry and events are unclear.
The pipeline is about 85 percent constructed, and the only remaining work to be completed in North Dakota is the section under Lake Oahe.
The Standing Rock Sioux Tribe and environmental activists claim the pipeline construction will pollute nearby water sources and destroy the tribe's sacred sites.
The Associated Press and NBC contributed to this report.