CANNON BALL, N.D. — With the green light from the federal government, the company building the Dakota Access oil pipeline said Wednesday it plans to resume work immediately to finish the long-stalled project. Opponents of the $3.8 billion project meanwhile protested around the country in an action some dubbed their “last stand.”

The Army on Wednesday granted the developer of the four-state oil pipeline formal permission to lay pipe under a Missouri River reservoir in North Dakota, clearing the way for completion of the project.

“We plan to begin immediately,” Vicki Granado, a spokeswoman for developer Energy Transfer Partners, said in an email to The Associated Press on Wednesday night.

Work had been stalled for months because of opposition by the Standing Rock Sioux, but President Trump last month instructed the Army Corps of Engineers to advance pipeline construction.

The tribe fears a pipeline leak could contaminate its drinking water. ETP says the pipeline is safe.

“Now, we all need to work together to make sure the project is completed safely and with as little disruption to the community as possible. This has been a very difficult issue for everyone who lives and works in the area,” U.S. Sen. John Hoeven, a North Dakota Republican, said in a statement announcing that the final easement had been granted.

Some members of the Standing Rock Sioux, which has been at the center of the debate for nearly a year, urged “emergency actions” via social media. The Indigenous Environmental Network told people to target fuel-transportation hubs and government buildings and to expect violence and mass arrests.

Protesters posted an online list of about 50 events nationwide. There were large rallies, including one outside the White House, and smaller ones, such as in Des Moines, Iowa.

A group of protesters in Chicago targeted a bank, and another group went to an Army Corps of Engineers office in New York City but was asked to leave when it started filming without a permit. Several people were arrested for blocking public access to a federal building in San Francisco.

“Today begins the next phase of mass resistance to Donald Trump’s toxic Dakota Access pipeline,” said Dallas Goldtooth, executive director of the Indigenous Environmental Network. “This is our land, our water, our health, and our culture at stake – and if Donald Trump thinks we will give all of that up without a fight he is wrong.”

At a North Dakota encampment that’s been the focus of the pipeline battle for months, the mood was tense and a reporter was ordered to leave.