Police arrested 56 people at the courthouse demonstration, St. Louis police spokeswoman Schron Jackson told CNN.
Protest groups have condemned the police response to protesters who gathered along West Florissant Avenue on Sunday.
Monday night, there is another man critically injured, who police said shot at them first.
St. Louis County Police officials said safety had been compromised at the evening’s protests by crowds that began to pelt officers with debris. Some demonstrators responded by throwing water bottles and other objects at police, and nine people were arrested. That total later increased to “approximately 23”. Police said Harris was being treated in hospital.
In a statement, St. Louis County executive Steve Stenger said county police would immediately take charge of “police emergency management” in Ferguson and surrounding districts.
Jeff Roberson/AP County police said they arrested protesters who wouldn’t disperse from the roadway.
Ferguson has seen months of violent protest since the fatal shooting of 18-year-old Michael Brown by a white police officer a year ago.
All four officers were in an unmarked van, wearing protective vests and escaped injury. The face-off was shattered by bursts of gunfire, sending police crouching behind their patrol cars and demonstrators scurrying for cover.
“They say what we are doing is excessive, but the death of Mike Brown brought an issue that has been swept under the rug to the light, so I like it, it’s powerful”, said a former Ferguson resident who goes by the name Mal Stiff.
Outrage over the police killings of Brown and other unarmed African Americans has been channeled into a sustained nationwide movement, with the social media hashtag #BlackLivesMatter becoming its rallying cry.
According to the probable cause statement, the detectives saw Harris running across West Florissant firing a handgun while gunfire was being exchanged between two groups of people around 11:20 p.m.
Police Chief Jon Belmar tried to de-escalate the tension by speaking with some demonstrators, who then moved from the street to the sidewalks.
The tense standoff continued till Tuesday morning with protesters hurling bottles and rocks at police which, in turn, used pepper spary to control the agitated crowd.
In Ferguson, a few merchants said they were ready to protect their businesses with firearms, while store owners pleaded for calm. Clergy members prayed in front of the building and spread oil on it, saying they were anointing it. The protesters were being processed and released “as quickly as can administratively be accomplished by the United States Marshal Service”, U.S. Attorney Richard Callahan said.
A father holds his son as he takes part in a demonstration in Ferguson, Mo., Monday night.
Police said there were no shootings, shots fired, burglaries, lootings or property damage.
The street has been the focus of demonstrations marking the anniversary of the shooting death of 18-year-old Michael Brown.
Belmar said the suspect who fired on officers had a semi-automatic 9 mm gun that was stolen last year from Cape Girardeau, Missouri.
Belmar said it is common to use plainclothes officers.
Wilson retired from the Ferguson police.
The US Department of Justice, however, also found a pattern of excessive police force against unarmed black citizens.