FERGUSON, Mo. — For four nights in a row, they streamed onto West Florissant Avenue wearing camouflage, black helmets and vests with “POLICE” stamped on the back. They carried objects that doubled as warnings: assault rifles and ammunition, slender black nightsticks and gas masks.

They were not just one police force but many, hailing from communities throughout north St. Louis County and loosely coordinated by the county police.

Their adversaries were a ragtag group of mostly unarmed neighborhood residents, hundreds of African-Americans whose pent-up fury at the police had sent them pouring onto streets and sidewalks in Ferguson, demanding justice for Michael Brown, the 18-year-old who was fatally shot by a police officer on Saturday.

When the protesters refused to retreat from the streets, threw firebombs or walked too close to a police officer, the response was swift and unrelenting: tear gas and rubber bullets.