The order was issued for the St. Louis suburb and surrounding areas amid already tense relations between residents and police after officers shot and critically wounded an 18-year-old man in an exchange of gunfire that marred what had been a day of peaceful demonstrations.
Prosecutors charged the man, Tyrone Harris, who was in critical condition in hospital, with four criminal counts, including "assault on law enforcement" and shooting at a motor vehicle. His bond was set at $250,000.
St. Louis County Executive Steve Stenger said he made the state of emergency declaration because of "the potential for harm to persons and property." It marked another chapter in the turmoil that has gripped Ferguson since Michael Brown, an unarmed black teenager, was shot dead by white officer Darren Wilson a year ago.
On a day of civil disobedience called by activists to protest the shooting of Brown and other unarmed black men across the United States by police, several people were arrested as they jumped barricades at a courthouse in St. Louis, news video reports showed.
Clergy and civil rights groups led the rally of more than 100 people through city streets, shouting, "This is what democracy looks like" and "Black lives matter."
The death of Brown and a grand jury's decision to spare the white officer from criminal charges led to a wave of demonstrations that boiled over into rioting and arson at times and spawned sympathy rallies across the country.
Brown's death also prompted greater scrutiny of racial bias within the U.S. criminal justice system, giving rise to the "Black Lives Matter" movement that gained momentum from other high-profile killings of unarmed minorities by white police in cities such as New York, Baltimore, Los Angeles, Cincinnati and most recently Arlington, Texas..
NIGHT OF VIOLENCE
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," he said adding, "We can't afford to have this kind of violence. It's untenable at this point".
Harris' 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.
Activist groups, meanwhile, said the plain clothes officers who shot Harris should never have been deployed to the scene.
"It was a poor decision to use plain clothes officers in a protest setting because it made it difficult for people to identify police officers, which is essential to the safety of community members,” said Kayla Reed, a field organizer with the Organization of Black Struggle.