St. Louis County Police Chief Jon Belmar told reporters a 41-year-old officer from his department was struck in the shoulder and a 32-year-old officer from the nearby Webster Groves Police Department was hit in the face around midnight as the crowd was starting to break up.
The shooting of the officers, who were in serious condition at a local hospital, was the latest incident in months of turmoil in the St. Louis suburb of Ferguson, which has been at the center of an intense national debate over police use of force, particularly against black men, since a white officer killed an unarmed black teenager there in August.
"These police officers were standing there, and they were shot, just because they were police officers," said Belmar.
No description of the culprit was given. "I don't know who did the shooting, to be honest with you," added Belmar, who said his assumption was that the shooter was "embedded" among the protesters.
However, protesters at the scene 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, according to Reuters.
Several dozen protesters had gathered in front of the Ferguson police department earlier on Wednesday night, just hours after the city's police chief, Thomas Jackson, announced his resignation.
Wednesday's demonstration had been tense but peaceful throughout the night. Several dozen people attended, and at one point demonstrators hung a flag with the words "Racism lives here" over a silhouette of the St. Louis skyline and drew chalk outlines in the police department parking lot.
The crowd had appeared to be dwindling when the shots rang out around midnight, turning a scene of relative quiet into pandemonium. Marciay Pitchford, 20, was among the protesters. She told The Associated Press the protest was mostly peaceful until she heard the shots ring out.
"I saw the officer go down, and the other police officers drew their guns while other officers dragged the injured officer away," she said. "All of a sudden, everybody started running or dropping to the ground."
Protesters had called for Jackson's removal since the fatal shooting of Michael Brown by white Ferguson police officer Darren Wilson on Aug. 9, 2014.
Jackson took nearly a week to publicly identify Wilson as the shooter and then further heightened tension in the community by releasing Wilson's name at the same time as store security video that police said showed Brown stealing a box of cigars and shoving a clerk a short time before his death.
The killing triggered nationwide protests and drew scrutiny to police use of deadly force, especially against black men.
Neither a grand jury proceeding nor a Department of Justice investigation led to any charges against Wilson. But the DOJ found damning evidence of widespread racism at Ferguson's police department, court and jail. The report said that officers routinely targeted African-Americans for arrests and ticketing to boost city revenue through fines.
Jackson's departure was the latest in a string of officials who stepped down after the scathing report. City Manager John Shaw and Municipal Judge Ronald Brockmeyer resigned earlier this week. Three other employees of the police department and municipal court left their jobs last week after the release of the report.
U.S. Attorney General Eric Holder said last Friday that the Department of Justice would use its full authority to demand police reforms in Ferguson, including possibly dismantling the department.
Al Jazeera and wire services
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