St. Louis County Police Department spokesman Brian Schellman confirmed the shooting to AFP but declined to detail the officer’s condition, saying he was en route to the scene.
Local television, however, said the officer was shot in the arm.
Ferguson has seen large protests take place since Michael Brown, 18, was shot dead on August 9 by a white police officer.
The college-bound teen was shot at least six times by police officer Darren Wilson and his body was left in the street for several hours before it was removed.
Violence rocked Ferguson — a St. Louis suburb of 21,000 with an African-American majority and an overwhelmingly white police department and town council — prompting Missouri Governor Jay Nixon to briefly call in the National Guard to quell protests.
Some demonstrators complained that police used undue force against peaceful protests.