FERGUSON, Mo. — A day of peaceful protest commemorating the killing of Michael Brown, an unarmed black 18-year-old, by a white police officer one year ago ended in gunfire late Sunday as multiple shots rang out on West Florissant Avenue, the commercial district hit by rioting last summer.

A St. Louis County police officer was involved in the shooting, which occurred at about 11:15 p.m., after coming under heavy gunfire, the department said in a post on Twitter. Another department post said, “At least 2 unmarked cars took shots.”

About 100 people who had been milling about in the street scattered, seeking cover behind cars. Police officers sought protection behind their riot shields and drew their weapons. The streets cleared as an eerie silence took hold, the only sound coming from a helicopter overhead. There was no immediate word from the police on injuries.

The event during the day drew a range of political leaders, national activists and ordinary people who said they felt a connection to the events of a year ago. With the writer and academic Cornel West behind them, Mr. Brown’s family stood over the freshly repaved patch of Canfield Drive where Mr. Brown died. Relatives of Eric Garner and Oscar Grant, two other black men who died in confrontations with the police in Staten Island and in Oakland, Calif., also attended.