Police stand guard moments after gun shots were fired outside the City of Ferguson Police Department and Municipal Court in Ferguson, Missouri, early March 12, 2015.[Photo/Agencies]

"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.

Protesters at the scene however 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 added.

Protesters had called for Jackson's removal since the fatal shooting of unarmed black 18-year-old Michael Brown by white Ferguson police officer Darren Wilson in August.

The killing triggered protests around the country and drew scrutiny to police use of deadly force, especially against black men.

Neither a grand jury nor the federal probe led to charges against Wilson.

Jackson was the latest in a string of Ferguson officials to resign in the week since a scathing Justice Department report found widespread racially biased abuses in the city's policing and municipal court.