Police and about 400 people who were protesting the Dakota Access Pipeline clashed Sunday evening as demonstrators lit cars on fire and police launched tear gas and water at the crowds.
A live stream from the North Dakota location showed a chaotic, loud scene full of screaming crowds and honking cars.
Protesters were attempting to cross the Backwater Bridge and go north on Highway 1806, according to the Morton County Sheriff's Department, which described Sunday's events as an "ongoing riot."
Protesters set fire to two trucks and several parts of the bridge, police said. On Sunday night, police released a statement saying that the protesters "attempted to flank and attack the law enforcement line from the west," describing their actions as "very aggressive."
Officers tried to disperse the crowds.
Some decried the use of water on the crowd on a cold night when temperatures fell to 28 degrees Fahrenheit.
The physicians and tribal healers with the Standing Rock Medic and Healer Council called for " the immediate cessation of use of water cannons" over concerns of hypothermia in the cold weather conditions. They criticized the use of water as "potentially lethal use of these confrontational methods against people peacefully assembled."
But police say the protesters are not peaceful and that water was used to put out fires as well as to control the crowds.
"There are multiple fires being set by protesters on the bridge and in the area of the bridge," said Donnell Hushka, spokeswoman for the Morton County Sheriff's Department. "We have fire trucks on the scene they are using their fire hoses to put out the fires, wet the land around so fires don't spread and they are also using water as crowd control."
One person has been arrested, police said.
Protests have simmered for months, spawning bitter clashes over the 1,172-mile oil pipeline that would span from North Dakota to Illinois.
Why protests are happening
Protesters say the Dakota Access Pipeline will threaten the environment and destroy Native American burial sites, prayer sites and culturally significant artifacts. The Standing Rock Sioux Tribe says construction of the pipeline -- which is currently slated to run under the Missouri River -- could affect its drinking water supply and put communities living downstream "at risk for contamination by crude oil leaks and spills."
Multiple groups have joined the protests over the months.
Protesters appear to be digging their heels in for the winter by building structures in a protest camp without a permit, said the Morton County Sheriff's Department.
"Their actions are both illegal and likely insufficient to protect them from the elements," said Morton County Sheriff Kyle Kirchmeier. "We've seen that many of these protestors are not from North Dakota and may not be familiar with the harshness of our winters, and we urge them to leave the camps and seek appropriate shelter for their own health and safety."
Bernie Sanders tweeted that the president "must protect the safety of Native Americans and their supporters who have gathered peacefully to oppose the Dakota Access Pipeline."
Status of the pipeline
Last week, the Army Corps of Engineers announced it had delayed construction work on the controversial pipeline to hold further "discussion and analysis" with Standing Rock Sioux Tribe. But the companies behind the Dakota Access Pipeline slammed the latest decision as "lacking legal or factual justification."
Energy Transfer Partners and Sunoco Logistics Partners took legal action, asking a federal court to allow them to complete the pipeline.
Dakota Access is a $3.7-billion project that backers have touted as the safest and most efficient way to transport oil, rather than using rail or trucks.