Police officers in riot gear initially watched the mayhem from a distance. But by 5 p.m. — in scenes that have become common this summer in a normally peaceful city — they were firing tear gas at protesters and pinning some to the ground.

The protesters, meanwhile, were throwing bricks and gasoline bombs into the road to impede police charges, and setting fires in the streets.

As night fell, hundreds of protesters decamped to a shopping mall near the Yuen Long subway station for a sit-in. The authorities had closed the station earlier in the day.

One protester at the mall, Fanny Yeung, 38, said she and her family had spent more than an hour traveling there because of train closings. But she said it was worth it.

“This is a battle between justice and rule by terror,” she said. “We have to come out to say we will always stand on the side of justice.”