A Concordia student already suing the city over alleged harassment by police says she was pushed to the ground at a protest Friday after identifying an undercover officer. 

Katie Nelson, who says she suffered a concussion and bruises in the incident, claims in her lawsuit that Montreal police have targeted and harassed her at protests. She has been handed more than $6,000 in fines by police.

Nelson claims she recognized one of the police officers named in the lawsuit at Friday's protest.

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A photo posted to Facebook allegedly shows Katie Nelson on the pavement after being pushed by Montreal police. (Martin Ouellet/Facebook)

After the officer noticed her, he began to swear at her in English and French, she told CBC News.

Nelson says she started to scream and yell, "This is a cop."   

Soon afterwards another officer slammed into her back, knocking her to the street, she claims.

"I had been thrown quite far," she said. 

Nelson said her friends dragged her to the sidewalk. She was then taken to the hospital in an ambulance.

"It was a just a very violent police intervention." she said.  

Martin Ouellet, an independent photographer, took photos of the confrontation.

He corroborates Nelson's story, saying he saw another man wearing a mask push her to the ground with both hands.

Ouellet also believes the man was an undercover officer.

Montreal Police Cmdr. Ian Lafrenière confirmed that undercover agents were at the protest, which he said is normal procedure.

Police are looking into the incident, including reviewing videos and photos taken at the scene, but Lafrenière said it remains unclear what happened.

Nelson said her lawsuit is expected to be heard this summer.