(Updates with details from a letter in paragraphs 9-11)
By Terray Sylvester
BISMARCK, N.D., Nov 25 (Reuters) - More than 30 activists protesting plans to run an oil pipeline beneath a lake near a North Dakota Indian reservation were arrested on Friday at a retail mall during a rally timed to coincide with the busiest shopping day of the year.
The rally was held at the main shopping center of North Dakota's capital city on "Black Friday" in a bid to draw more attention to a pipeline project that critics say poses a threat to water resources and sacred Native American sites.
The demonstrators, including members of the Standing Rock Sioux Tribe, walked into the Kirkwood Mall in downtown Bismarck and formed a prayer circle just inside the entrance, defying demands by mall management that they leave the premises.
About 100 protesters gathered at the mall at shortly before 1 p.m. and at least 33 people were taken into custody for trespassing on private property after they disregarded repeated orders to disperse, police said.
The incident marked the latest in a string of protests against the $3.8 billion Dakota Access Pipeline, being built to carry Bakken shale oil from North Dakota to Illinois en route to U.S. Gulf Coast refineries.
The 1,172-mile (1,885-km) project is mostly complete except for an especially controversial segment that is supposed to run under Lake Oahe, formed by a dam on the Missouri River, about a half mile from the Standing Rock Sioux reservation.
The Obama administration in September postponed final approval of a permit required to allow tunneling beneath the lake, a move intended to give federal officials more time to consult with tribal leaders. But the delay also led to escalating tensions over the project.
Confrontations between law enforcement and protesters turned violent again last weekend when police used water hoses in sub-freezing weather in an attempt to disperse about 400 activists near the proposed tunnel excavation site.
To protect the general public from violent confrontations, federal authorities on Dec. 5 will close off land north of the Cannonball River to the public and will set up a free speech zone on the south side of the river, Standing Rock Sioux Tribe officials posted on social media on Friday, citing a letter from the U.S. Army Corps of Engineers.
The land being closed off does not include the Sacred Stone Camp, a point of resistance to the pipeline, south of the river.
The authenticity of the letter from the Army Corps could not be verified by Reuters. Corps officials were not immediately available for comment. (Writing and additional reporting by Steve Gorman in Los Angeles and Brendan O'Brien in Milwaukee; Editing by Himani Sarkar)