Read the latest on the Dakota Access pipeline controversy: Dakota Pipeline Was Approved by Army Corps Over Objections of Three Federal Agencies

A groundswell of Native American activists has temporarily shut down construction on a major new oil pipeline with an ongoing protest that has drawn around 1,200 people to Cannon Ball, N.D.

Construction workers walked away from their bulldozers Monday after protesters surrounded the equipment and called for an end to construction of the Dakota Access pipeline. A group of protesters on horseback also staged a mock charge toward a line of law enforcement officials guarding the site, and the county sheriff alleged others have fired guns and set off pipe bombs.

The $3.8 billion pipeline at the heart of the protest would carry about half a million barrels of crude oil per day from the Bakken oil field to Illinois where it would link with other pipelines to transport the oil to Gulf Coast refineries and terminals.

The protest was staged at a spot where the pipeline would pass beneath the Missouri River, just upstream from the Standing Rock Sioux reservation, a community of 8,500 along the Missouri River in North and South Dakota.  

Protesters from dozens of tribes across the country are now camping in tents, tepees and mobile homes at the Sacred Stone Camp a mile and a half from the construction site. A video shows a second, more recently established campsite, the Red Warrior Camp.

“We have to be here,” David Archambault II, the chairman of the Standing Rock Sioux tribe, who was arrested at the site last week, said in a statement. “We have to stand and protect ourselves and those who cannot speak for themselves.”

The pipeline’s builder, Energy Transfer Partners, said through a spokesperson that it is “constructing this pipeline in accordance with applicable laws, and the local, state and federal permits and approvals we have received.”

“This is an important energy infrastructure project that benefits all Americans and our national economy,” it said. The company did not respond to a request for additional comment.

The Standing Rock tribe, one of the poorest communities in the nation according to 2010 census data cited by the tribe, relies on the Missouri River for drinking water, irrigation, fishing and recreation, and for cultural and religious practices. The reservation covers about 3,600 square miles along the river.

“An oil spill would represent a genuine catastrophe for the people who live there,” said Jan Hasselman, an attorney with Earthjustice, an environmental organization that filed a lawsuit on behalf of the Standing Rock Tribe against the U.S. Army Corps of Engineers, which approved the pipeline. “It isn’t just cultural and religious, it’s their economic lifeblood.”

The suit alleges the pipeline violates the Clean Water Act, the National Environmental Policy Act and the National Historic Preservation Act.

Protests against the pipeline have been ongoing since April. In July, a group of roughly two dozen from the Standing Rock tribe completed a nearly 2,000-mile relay from Cannon Ball to Washington, D.C. They delivered a petition with 150,000 signatures to the Corps calling on it to halt construction of the project.

On July 25, the Corps approved construction of the section of the pipeline upstream from the Standing Rock reservation, and ground was broken on August 10. Protests at the site started small, with about 50 people and grew to an estimated 1,200 on Wednesday, according to Morton County Sheriff Kyle Kirchmeier.

Mekasi Horinek Camp, a member of the Ponca Nation of Oklahoma and coordinator of the environmental group Bold Oklahoma, claimed the Sioux bands “haven’t come together in this traditional way since the Battle of the Little Big Horn.”

“It’s an historical time,” he said, “and just a beautiful thing to be a part of.”

As the number of protesters at the construction site and the nearby Sacred Stone Camp swelled, tensions between activists and law enforcement rose.

Protesters came on horseback Monday and a video shows what appears to be a mock charge aimed at law enforcement officials who had formed a line along a steep embankment near the entrance to the construction site. The video shows horses charging toward the officers and pacing in front of their line, directed by activists who yelled a “war whoop,” or battle cry.

Neither the horses nor the protesters made physical contact with the officers, according to Montgomery Brown, a member of the Standing Rock Sioux tribe who was at the protest. The officers, however, appear visibly frightened in the video and quickly scrambled up the embankment away from the horses.

Camp said the demonstration was part of a traditional ceremony that brings in the spirit of horses, and Brown called it a traditional way of introducing warriors from separate tribes. The officers were notified of the ceremony ahead of time, Camp said, and were asked to back up to give the horses space.

Kirchmeier said the protest had become “unlawful” as his officers reported incidents of shots being fired, pipe bombs, vandalism and assaults on private security personnel. Construction on the pipeline near Cannon Ball has been “discontinued for the time being,” Kirchmeier said.

Protesters denied those allegations. “Firearms and weapons are not allowed at the Sacred Stone Camp and our security has done an exemplary job at maintaining safety amongst the crowd,” according to a statement released by Sacred Stone Camp protesters with the groups Honor the Earth and the Indigenous Environmental Network. “As our camp was established on an act of prayer, we are committed to nonviolence.”

“We are disappointed that there are those who will put the lives of others in jeopardy,” the Energy Transfer Partners‘ spokesperson said. “We will continue to put the safety of our workers and those who live in the area as our top priority.”

The Army Corps of Engineers declined to comment on the project citing the ongoing litigation.

While the protest in Cannon Ball is primarily Native Americans, ranchers are challenging the project elsewhere along its 1,168-mile path. On August 9, lawyers representing 14 Iowa landowners filed a motion to halt construction of the pipeline across their property. The suit challenged Dakota Access’s use of eminent domain to seize land for what it says is private use.  

Over the past year, protests  against fossil fuel infrastucture projects nationwide have increaseed, and at least 24 dozen projects have been rejected or canceled for myriad reasons, the most prominent among them the Keystone XL pipeline.

Protesters at the Sacred Stone Camp said they are hopeful that a federal court will rule in their favor when their case is heard on August 24. In the meantime, they are planning to continue their protests.

Brown, the Standing Rock member, who is also a former Navy medic, said he is seeking additional medical professionals to help ensure the demonstrations can last.

“One of my main concerns right now is either pneumonia or tuberculosis since we are camped so close around each other,” Brown said. “From a medical standpoint, you are going to need a lot of staff for these people to self-sustain.”