St Louis, Missouri: Two police officers have been shot, one in the face and the other through the shoulder, outside the Ferguson police department where a large group of protesters gathered following the resignation of Ferguson police chief Thomas Jackson.

A scathing Justice Department report of entrenched racism in the Ferguson police force led to the chief's departure.

At a press conference in Ferguson, St Louis County Police Chief Jon Belmar confirmed one of the shot officers was from the St Louis County Police and the other was from the Webster Groves Police Department.

Mr Belmar said the men, aged 32 and 41, were conscious and had been treated at a local hospital.  They were released on Thursday morning.

"Police officers were standing there and they were shot," Chief Belmar said. "This is really an ambush, is what it is. You can't see it coming, you don't understand that it is going to happen and you are basically defenceless."

The Webster Groves officer was shot under his right eye at the high point of the cheek and through a riot helmet he was wearing, Chief Belmar said.

"The St Louis County officer was struck right here on the shoulder and the bullet came out the middle of his right back, between his scapula and his spine," he said.

Chief Belmar said the protest had been rowdy earlier in the evening, with protesters closing South Florrisant Road in front of the Ferguson Police Department and two "code 1000" emergency calls from police drawing available officers from around St Louis.

"Approximately 150 protesters were located on the roadway, 15 police officers on the scene. At 9 o'clock Ferguson makes one arrest of a protester on the roadway on the charges of failing to comply being on the roadway, endangering the welfare of a child and resisting or interfering with arrest. Protesters leave the roadway ... Eventually we end up with 69 police officers that respond."

However, the protesters had begun to disperse when, just after midnight, at least three shots came from the north or north-west of the police department.

There were about 75 protesters spread out across a wide area when the police officers were targeted, Chief Belmare said. The shots were fired from an estimated range of 115 metres.

"Those shots immediately strike two officers who are standing next to each other," he said.

"What I'd like to underscore is that these officers are also standing in a line of about of at least 20 to 25 officers," Belmar said. "It is something that is difficult to guard against when you have a group of officers standing in a large group, and you have gunfire directed against them."

The police did not return fire.

"All of a sudden, I heard at least four or five shots rang out," a witness, Markus Loehrer, told CNN. "It took me at least 30 seconds of watching before I realised there was an officer down. We are not there to shoot cops, we don't like violence. So we did what anybody would do, we ran away."

CNN reported at least one scuffle broke out between demonstrators, and officers were taking cover behind cars with their firearms drawn.

Officers with shields and rifles making their way up hill across from PD. Searching for shooter — John Eligon (@jeligon) March 12, 2015

DEVELOPING: Two police officers reportedly shot outside the #Ferguson, MO Police Department; from KSDK-TV. — KAKE News (@KAKEnews) March 12, 2015

#BREAKING: Shots came from a house up the hill across from police station in #Ferguson. 2 police officers shot - @susan_weich — Al Boe (@AlBoeNEWS) March 12, 2015

#BreakingNews: Two police officers shot at #Ferguson protest http://t.co/FFrbOxRkC8 via @dailykos More violence in this tiny town... — Angie in WA State (@AngieinWAState) March 12, 2015

The crowd of protesters has dispersed since the shooting, and police cordoned off the area at about 2 am.

The St Louis County Police Department's homicide unit was on site and attempted to interview people but many were reluctant to talk. Dozens of officers from around the region also arrived. Several held rifles and crouched behind cars, walls and gates.

A group of officers in tactical gear was investigating the hill across from the police station where witnesses said the shots had come from.

Mid-morning, a SWAT team was seen raiding a brick bungalow about four blocks from the police station. Neighbours said three people had been taken away, including one woman in handcuffs. CNN showed images of the SWAT team trying to break into the roof of the home in search of a suspect.

Protest leaders doubted the shooter, who is thought to have been armed with a handgun, was among their ranks.

"There are some who have been waiting for any moment to attempt to vilify and discredit protesters," one leader, DeRay Mckesson, said on Twitter. "But the shooter did not stand with us," he said, adding that the local office of the Federal Bureau of Investigation should take over the case.

The embattled suburb has been the subject of angry protests and a national debate about race and law enforcement since a white police officer killed an unarmed black teenager on August 9 last year.

The Justice Department commenced an investigation into the Ferguson police force shortly after then-police officer Darren Wilson shot Michael Brown.

In a report handed down on Wednesday, the department outlined the racial bias in an overwhelmingly white force. It found local police targeted Ferguson's African-American majority.

Chief Jackson resigned on Wednesday, effective March 19. He is the sixth Ferguson official to resign. Others who have already resigned included Ferguson's municipal court judge and its city manager.

With Reuters