Police armed with clubs hurled tear gas and deployed officers with dogs to curb the violence which erupted yesterday in the suburb in Missouri, media report.
But local law enforcement officers were quickly overwhelmed and had to urgently call for backup from nearby communities, KSDK TV 5 said.
Video posted by the St Louis Post-Dispatch showed a QuikTrip gas station convenience store plundered and set on fire.
Looters also raided a Walmart and several other smaller stores, and set fires elsewhere, the newspaper reported.
Details of Brown's death differ. A witness identified as Dorian Johnson told KMOV News 4 that he was walking with Brown when a police officer confronted them and drew his weapon.
The officer shot Brown, who "turned around and put his hands in the air," Johnson said. "He started to get down and the officer still approached with his weapon drawn and fired several more shots."
St Louis County Police Chief Jon Belmar however said at a televised press conference that Brown was killed after physically assaulting a police officer and struggling to get his weapon.
The incident brings to light the tension between the mostly white local police force and the African-American community, the Post-Dispatch said.
The St Louis County chapter of the NAACP civil rights group said in statements that it has launched an investigation, and is calling for a meeting Monday "in response to the recent murder of Mike Brown".
Chapter president Ester Haywood also called on the community to "settle down" in an interview with KSDK 5 TV station.
Brown's grief-stricken mother, Lesley McSpadden, told KMOV TV that her son had just graduated from high school and planned to attend college.