A man was killed early Monday in Louisville, Ky., when police officers and National Guard troops were breaking up a group of protesters. Someone in the crowd fired at them, and the troops and officers fired back, the authorities said.

The shooting happened just after midnight, the authorities said, when officers from the Louisville Metro Police Department and the National Guard soldiers were enforcing the city’s curfew. The group of protesters had gathered outside a market in a neighborhood west of downtown.