Anonymous, the loosely organized group of international hackers, said on Twitter that it had broken into Ferguson’s municipal computer system. It released details about city workers and posted photos of Jon Belmar, the chief of the St. Louis County police who is conducting the investigation into the shooting, as well as his wife, son and daughter. It also posted his address and phone number. The group threatened to bring down city, county and federal networks if the police overreacted to rallies and protests.

On Wednesday night, scores of police officers in riot gear and in armored trucks showed up to disperse protesters who had gathered on the streets near the scene of the shooting. Some officers perched atop the vehicles with their guns trained on the crowds while protesters chanted, “Hands up, don’t shoot.” A police spokesman said that some demonstrators had thrown Molotov cocktails at officers and that some had tried to set fires. The police used tear gas on demonstrators, and some protesters said rubber bullets had been fired at them. Police said one officer appeared to have suffered a broken ankle after being hit by a brick.

The police made more than 10 arrests. Among those arrested was Antonio French, a St. Louis alderman, who had been documenting the protests on social media, his wife said on Twitter.

Two reporters covering the protests also said they had been arrested inside a McDonald’s for trespassing and later released without charges or an explanation. The reporters, Wesley Lowery of The Washington Post and Ryan J. Reilly of The Huffington Post, both said they had been handled roughly by the police.