Police, political leaders and civil rights activists have sought to damp down tensions after two police officers were shot in front of the police department during a protest in Ferguson, which has become a symbol of tensions between police and African Americans.

The shootings took place during a protest that unfolded following the resignation of police chief Tom Jackson in the wake of a damning US Justice Department report. The gunfire began just as the small crowd of protesters began to break up.

Later, several people were taken in for questioning after a special weapons and tactics team converged on a Ferguson home near the shooting site, but they were released, and no arrests were made.

The shots were believed to come from a handgun across the street from the police department, which has been a national focal point since the fatal August 9 shooting of 18-year-old Michael Brown, who was black and unarmed, by a white police officer.

Hours after the shooting about 50 people gathered at a public plaza downtown near the police station for a vigil. The group sang spirituals, prayed for peace and expressed sympathies for the injured officers.

Later, a larger group marched, chanted and beat drums in the street in front of the police department. Some were demanding the resignation of Ferguson mayor James Knowles III. About 20 officers were visible outside the station.

Most of the protesters disbanded late in the evening. No arrests were made.

Tensions have been high in the St Louis suburb since August and escalated in November after a grand jury declined to prosecute officer Darren Wilson for Mr Brown's killing. Justice Department investigators concurred with the grand jury's finding in a report released March 4.

But a separate Justice Department report released that same day found racial profiling and bias in the Ferguson police force, and a municipal court system driven by profit, largely on the backs of black and low-income residents.

The shootings marked the first time in more than seven months of tension in Ferguson that officers were shot at a protest, and the bloodshed threatened to inflame the already fraught relationship between police and protesters just as the city seeks reforms.

Both officers suffered significant wounds but are expected to recover.