Activists who protested plans for an oil pipeline beneath a lake near an Indian reservation in North Dakota were arrested at a retail mall on Friday.

They timed their rally for Black Friday, the busiest shopping day of the year in the U.S., as Reuters and other outlets reported.

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Critics of the pipeline claim it poses a threat to water resources and sacred Native American sites.

The rally was held at the main shopping center in Bismarck, North Dakota’s capital city.

Among the demonstrators were members of the Standing Rock Sioux Tribe. They “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,” a report in Reuters noted.

Some 100 protesters gathered at the mall in the early afternoon — and at least 33 people were taken into custody. The charge was trespassing on private property after they disregarded repeated orders to disperse, according to local police.