HONG KONG—Protesters in helmets and gas masks clashed with riot police Saturday, as the city’s pro-democracy movement took a confrontational turn after nearly two weeks of relatively peaceful turnouts.

Tear gas engulfed the industrial neighborhood of Kwun Tong as protesters were more aggressive, blocking roads, surrounding the local police station and sawing down at least one video-surveillance pole, which protesters said could be used to spy on people.

Demonstrators...