The authorities say Ferguson police department is under lockdown.
Around midnight witnesses claimed that a few dozen demonstrators fled following the sound of gunfire shouting: "They hit a cop". Police could not be reached immediately for comment.
Both police officers have been taken away by ambulances after the gunman fired around five or six shots, local witnesses said on Twitter.
Ferguson, Missouri, has been mired in protests after a white police man shot unarmed black teen Michael Brown in 2014. The jury did not indict the officer.
Earlier in March, the US Department of Justice released a report regarding the enforcement practices of Ferguson. The paper found that the city police department and municipal court intentionally racially profiled black people and violated their rights.
The report also revealed several city administrative and judicial officials systematically assisting friends, colleagues, acquaintances, and themselves in eliminating citations, fines, and fees.
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