What began as peaceful demonstrations on Sunday night in Ferguson descended into chaos as volleys of gunshots rang out as police in riot gear tried to disperse protesters blocking traffic and smashing storefront windows along a street that was a flashpoint of riots last year after Brown, 18, was shot dead.

Missouri Governor Jay Nixon called Sunday's violence "a sad turn of events" carried out by a criminal element and appealed for peaceful rallies on Monday. But Nixon, who had deployed the National Guard to quell violence last year, did not make any mention of additional security for those rallies.

The county's order allows it to distribute money and material for emergency purposes "and to protect the health and safety of residents." It also allows police to perform "all duties necessary with respect to preservation of order."

Police said Sunday's gunfire began with two groups of agitators apparently shooting at each other before one gunman darted across a parking lot and was confronted by four officers who pulled up in an unmarked vehicle.

Police said that the suspect, Harris, then opened fire on the police vehicle and was badly wounded in the ensuing foot chase and exchange of gunshots with the four detectives.

The shooters "were criminals; they weren't protesters," St. Louis County Police Chief Jon Belmar said, estimating that 40 rounds had been fired in the exchange. "We can't sustain this as a community," adding, "It's untenable at this point".

Harris's father said his son did not have a gun.

"He was running for his ... life because someone was shooting at him," Tyrone Harris, Sr., said in a telephone interview from his St. Louis-area home.