The arrests were the only incident in an otherwise peaceful weekend of demonstrations in St. Louis to highlight the unresolved fatal shooting of 18-year-old Michael Brown in August. The shooting sparked sometimes violent demonstrations in predominantly black Ferguson in August.

St. Louis Police Chief Sam Dotson said at a news conference that early Sunday morning about 200 protesters gathered in a south St. Louis neighborhood where another black 18-year-old was killed by a white police officer recently. The group, including some protesters wearing masks, started marching toward a QuickTrip convenience store and tried to force open its doors, Dotson said.

“That was the first time (this weekend) we’ve seen individuals become disrespectful,” Dotson said.

Police assembled in riot gear and instructed the crowd to disperse, he said. About 50 of the marchers created a human chain by locking their arms and about half of them heeded the police warning and left..

“The people who were left there were people who made a conscious decision they wanted to be arrested,” Dotson said.

At least one officer was hit by a rock but was not seriously injured. Some pepper spray was used to get those arrested to comply with police, Dotson said. Police were in the process later Sunday of releasing those arrested after they were charged with unlawful assembly.

Community activists and organizers of the protests did not immediately respond to requests for comment on the police version of events.

The planned demonstrations began Friday afternoon with a march outside the St. Louis County prosecutor’s office, where protesters renewed calls for prosecutor Bob McCulloch to charge Darren Wilson, a white Ferguson officer, in the Aug. 9 death of Brown. A grand jury is reviewing the case and the Justice Department has opened a civil rights investigation.

Since Brown’s death, three other fatal police shootings of black men have occurred in the St. Louis area. The most recent involved an off-duty St. Louis officer who was working for a private neighborhood security patrol when he shot and killed 18-year-old Vonderrit D. Myers on Wednesday night.

The white officer, whose name has not been released, fired 17 rounds after police say Myers opened fire. Myers’ parents say he was unarmed, and many speakers at a weekend rally echoed those doubts.

Demonstrators stood outside Busch Stadium in downtown St. Louis, where the Cardinals were playing the San Francisco Giants this weekend in the National League Championship Series of professional baseball. The St. Louis Post-Dispatch reported that fans headed to the game mainly went around the protesters on Saturday night. Game 2 in the series was Sunday night.