Scores of officers dressed in riot gear walked in a wide line, sweeping protesters out of the area as face-to-face yelling matches broke out. Several vehicles, including at least one truck, were set ablaze. A standoff unfolded beside a bridge known as the Backwater Bridge, where protesters set fire to wooden boards and signs and held off the line of officers over many hours.
By Friday evening, officers said they had arrested at least 142 protesters on charges including engaging in a riot and conspiracy to endanger by fire and explosion. Protesters gathered near the bridge were refusing to leave, the authorities said.
Each side complained vehemently about violent tactics by the other. Officers said that protesters had attacked them with firebombs, logs, feces and debris. They acknowledged using pepper spray and beanbag rounds against the protesters, as well as a high-pitched sound device meant to disperse crowds.
In one case, the officers said, they used a Taser gun after a protester threw pepper in officers' faces. One woman who was being arrested, the authorities said, had pulled a gun out and fired at a police line. No one was hit by those shots, they said, though two officers had minor injuries after being hit by debris.
“It was peaceful, but it's not now,” Randez Bailey, a resident of Standing Rock, said of the protest. “We are the ones who have to live here. You all get to go back home.”
The confrontation has been brewing for months as Energy Transfer Partners tries to finish construction of the Dakota Access pipeline, which is to carry oil 1,170 miles from North Dakota to Illinois. Company officials contend that the pipeline will be a safer way to transfer oil. But Native Americans and environmental activists, many of whom have gathered here, say the $3.7 billion pipeline threatens the region's water supply and would harm sacred cultural lands and tribal burial grounds.
Even as crews here were continuing construction of the pipeline along private lands, all sides were awaiting a review by the Army Corps of Engineers on a crucial stretch of the proposed path, through Army Corps land and under the Missouri River.
The issue has sparked concern from environmentalists and politicians on social media.
And the intensifying clashes as law enforcement moved in on Thursday drew a renewed flurry of attention from organizations like Amnesty International, which said it was sending observers to monitor law enforcement's behavior, as well as from celebrities.
Kyle Kirchmeier, the Morton County sheriff, said protesters had been asked to move away from a campsite they had created on private land that is owned by Energy Transfer Partners, but had refused.
“It forced our hand,” Sheriff Kirchmeier said. By Thursday morning, he said, the authorities had given up on negotiations with the protesters and moved in to clear the area, where more than 200 people were gathered. The authorities, some of whom arrived in military-style vehicles, demanded over loudspeakers that people leave before they began moving in, searching tent to tent.
Protesters were not being asked to evacuate a second, larger camp that they have set up on federal land, a few miles away. The authorities said those who were swept off the private land would be permitted to stay in the second camp.
But tribal leaders said the land in question was tribal land, and called on federal authorities to step in and oversee the actions of local law enforcement — particularly given Thursday's sweep, which brought the total number of protesters arrested since August to 411.
“We need our state and federal governments to bring justice and peace to our lands, not the force of armored vehicles,” said Dave Archambault II, the chairman of the Standing Rock Sioux tribe. “We have repeatedly seen a disproportionate response from law enforcement to water protectors' nonviolent exercise of their constitutional rights.”