A protest turned violent when oil pipeline company bulldozers began to dig up land that Native Americans said included sacred ancestral sites. Video from the scene apparently showed company security officers threatening protesters with dogs.
Hundreds of Native Americans from tribes across the US have set up a camp in southern North Dakota near the first phase of construction of a four-state $3.8bn (£2.9bn) oil pipeline to be built by the Dallas-based Energy Transfer Partners.
The US Army Corps of Engineers approved the Dakota Access Pipeline in July allowing it to run under the Missouri River very close to the Standing Rock Sioux reservation.
Protesters fear the construction will disturb sacred sites and contaminate drinking water for the tribe and millions more and a pipeline leak would be an ecological disaster.
A video taken of the protest scene shows Native Americans becoming upset as bulldozers begin tearing up private land behind a barbed wire fence as corporate security guards with German shepherds try to keep demonstrators at bay. The guards appear to be encouraging the dogs to attack, and protesters show bite marks.
A statement from the local Morton County Sheriff's Department said protesters "broke down a wire fence" and marched onto private land where the bulldozers were operating, National Public Radio reported.
"According to numerous witnesses within five minutes the crowd of protesters, estimated to be a few hundred people became violent," the statement added. "They stampeded into the construction area with horses, dogs and vehicles." Sheriff Kyle Kirchmeier said it "was more like a riot than a protest."
Videos show some protesters were injured while the sheriff said four private security officers and two dogs were hurt.
Tribal spokesman Steve Sitting Bear said protesters reported that six people had been bitten by security dogs, including a young child, Associated Press reported. At least 30 people were pepper-sprayed, he said. Sheriff's spokeswoman Donnell Preskey said law enforcement authorities had no reports of protesters being injured.
There were no law enforcement at the site when the confrontation occurred, Preskey said. The crowd dispersed when officers arrived and no one was arrested, she said.
Standing Rock Sioux chairman David Archambault II said in a statement that construction crews removed topsoil across an area about 150 feet (46m) wide stretching for two miles (3.2 km).
"This demolition is devastating," Archambault said. "These grounds are the resting places of our ancestors. The ancient cairns and stone prayer rings there cannot be replaced. In one day, our sacred land has been turned into hollow ground."
The Dakota Access Pipeline is designed to transport crude oil from North Dakota into Illinois, and will run through those two states as well as South Dakota and Iowa. The 1,170-mile pipeline crosses 50 counties and three major rivers.
Demonstrators in Kansas City also held a protest to support the Sioux and protest the pipeline's threat to water resources and sacred sites. There were also protests in Wisconsin and Iowa.
The Standing Rock Sioux have filed a lawsuit in federal court in a bid to stop the construction, accusing the Army Corps of Engineers of violating the Clean Water Act and the National Historic Preservation Act.
Tribal surveyors have found several sites of "significant cultural and historic value" along the path of the proposed pipeline, according to the court papers.
The tribe was only recently allowed to survey private land north of the Standing Rock Sioux reservation, tribal preservation officer Tim Mentz said in court documents. Researchers found burial rock piles called cairns and other sites of "historic significance to Native Americans", according to the court action.
A federal judge is expected to rule within the week if construction can continue.