Missouri police have launched a manhunt for the gunman who shot two policemen on Thursday in what authorities called an "ambush" at a protest outside the Ferguson police station.

Police Chief Jon Belmar of St. Louis County said the two officers managed to survive through "God's grace." One of them sustained a bullet wound in the face. Belmar said investigators believed the triggerman used a handgun.

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The officers were not identified but investigators said one is a 32-year-old cop from Webster Groves and the second victim is a 41-year-old policeman from St. Louis County.

The first officer took a hit in the cheek. The other officer suffered a gunshot wound to his shoulder, and the bullet exited his middle back, Belmar said. Both were taken to a local hospital. Their injuries were described as "very serious," but not life-threatening.

The injured policemen were members of a two-dozen security cordon around the station where the police chief of Ferguson, Thomas Jackson, made public his resignation a couple of hours earlier.

Witnesses said they heard up to four shots several meters away from the station but no arrests had been made so far.

The protest outside the department began at around 5:30 in the afternoon on Thursday after Jackson quit. His resignation followed a grand jury ruling and a Department of Justice report that cleared former Ferguson police, Darren Wilson, of wrongdoing in the killing of 18-year-old Michael Brown in Ferguson in summer last year.

Another Justice Department report called out a profit-driven judicial system and extensive racial bias in the Ferguson police department.

Attorney General Eric Holder called the shooting of the two law enforcers as "inexcusable and repugnant." He launched a probe into the conduct of the Ferguson police department after an apparent racially-motivated shooting in August last year.

In a statement, Holder wished the officers a full and speedy recovery, adding his office is prepared to offer help in investigating the incident, including the attorney general's "investigative resources." 

Tagspolice, Ferguson Police, Ferguson, michael brown, Thomas Jackson, racial bias