FERGUSON, Missouri (Reuters) - St Louis County's police chief said on Thursday there were possible leads in the search for the gunman who shot two officers during a protest in Ferguson, Missouri, and in future similar situations, the police may shoot back.

Muzzle flashes were detected from about 125 yards (114.3m) away from the rally early on Thursday, just hours after the city's police chief resigned in the wake of a scathing United States Justice Department report finding his force was rife with racial bias, Chief Jon Belmar told a press conference. Both officers were wounded, one with a bullet lodged behind his ear, Mr Belmar said.

US Attorney-General Eric Holder said on Thursday that the shooting was a "heinous assault" that threatens reforms under way in the city.

The shooting of the officers, who were in serious condition at a local hospital, comes after months of turmoil in the St Louis suburb of Ferguson, triggered last August by the killing of an unarmed black teenager by a white policeman.

The death of 18-year-old Michael Brown thrust Ferguson into the centre of an intense national debate over law enforcement's use of force against minorities, particularly against black men, and led to months of protests nationwide.

On Thursday, a manhunt was underway after the two officers were hit by gunfire outside the Ferguson police headquarters during a protest rally staged hours after the resignation of Police Chief Thomas Jackson.

The two officers who were shot were not part of the Ferguson force, which is almost entirely white, while the majority of the city's residents are African-American.

A 41-year-old officer from the St Louis County Police was struck in the shoulder and a 32-year-old officer from the nearby Webster Groves Police Department was hit in the face.

"These police officers were standing there and they were shot, just because they were police officers," Mr Belmar told reporters early on Thursday.

"I have said all along that we cannot sustain this forever without problems," he said, referring to festering tensions in the city since Mr Brown's death last summer.

Mr Belmar said the officers, whom he did not identify, were both conscious and hospitalized.

Mr Benjamin Crump, an attorney representing Mr Brown's parents, said on Thursday the family condemned the shooting and insisted that a small number of people were responsible for any violence.

"Violence is never the solution," Mr Crump told CNN. "There may be a few people who are misguided or confused but in large part the majority of the protesters and the majority of Americans want justice."

The shooting of the officers comes less than three months after the fatal shooting of two New York City patrolmen on Dec 20 by a man who said he was seeking to avenge the killings of Mr Brown and a second unarmed black man in New York City. In both cases, grand juries decided against bringing criminal charges against the officers involved.

In Ferguson, the protest started peacefully on Wednesday but about two dozen officers clad in riot gear later faced off with demonstrators, and at least two people were taken into custody.

When gunshots rang out about midnight, the scene turned into pandemonium. Many of the few dozen demonstrators who were still at the scene fled, some of them screaming.

The line of police scrambled, with many taking defensive positions with weapons drawn and some huddling behind riot shields, according to a video published online.

Belmar said the shooter was among the demonstrators standing across from the officers. "I don't know who did the shooting, to be honest with you right now, but somehow they were embedded in that group of folks," he said.

But protesters at the scene said on social media that the shots did not come from where they were standing.

"The shooter was not with the protesters. The shooter was atop the hill," activist DeRay McKesson said on Twitter. "I was here. I saw the officer fall. The shot came from at least 500 feet away from the officers," he said.

Activists had called for the police chief's removal since the fatal shooting of Brown, but McKesson said many were not satisfied merely with Jackson's departure and also wanted to see the city's Mayor James Knowles step down.

Mr Jackson was the latest in a string of Ferguson officials to resign in the week the Justice Department report, which found that widespread racial bias had led to the abuse of African-Americans by the city's police and court.

The investigation found that Ferguson used the police as a collection agency, issuing traffic citations to black residents to boost city coffers through fines. As a result, a "toxic environment" took hold in the city, it said.

After the report, Mr Holder said the federal government would use its full authority to demand police reforms in Ferguson, including possibly dismantling the department.

The Ferguson rally was staged on the same night as a march to protest the killing of an unarmed man in Madison, Wisconsin.

About 1,500 people, some banging plastic pails or blowing whistles, marched on Wednesday in the state's capital city after last week's fatal police shooting of a young biracial man.