An online video of a police officer knocking a woman over, apparently without provocation during the calm aftermath of a NCAA riot in Tucson, has University of Arizona students furious and the police department investigating the matter.

A crowd had assembled on University Boulevard in Tucson Saturday, following the school's loss to the University of Wisconsin in the National College Athletic Association men's basketball tournament. Beer bottles were thrown at police, who responded with pepper spray and rubber bullets.

But the riot had calmed down when the incident, below, was captured on video.

"She was just trying to get by the cops," Andrew Brown, a photographer who was on the scene, told Mashable. "He perceived her as a threat and decked her, out of nowhere. There wasn't any verbal warning, no 'Hey, stay back.'"

The Tucson Police Department did not respond to a question about the identity of the officer, but they told the Arizona Daily Star that they know about the video, and that its internal-affairs department is looking into the matter.

Officers had blocked off some streets to prevent the riot from spreading, but Brown said they didn't account for people who were trying to get from those streets back to their cars or homes, and were forced to walk through the line of police.

Phoebe Landolt, who shot the video, told a similar version of events to Daily Star.

"There were kids misbehaving, and there were kids who got arrested who I'm sure totally deserved it," Brown said. "But ... it was one of the most aggressive things I've seen in person."

The incident sparked outrage on Twitter:

Police brutality at the University of Arizona for real. Watch the videos people! — William Hansen (@HillyBansen0520) March 31, 2014

The video of the cop punching the girl from Arizona in the face is police brutality at it's finest, somebody should've attacked the cop — Christopher Coleman (@Chris11Coleman) March 31, 2014

"A dispersal order was given to the crowd numerous times in both English and Spanish, telling the crowd that the Tucson Police Department had declared their gathering to be an unlawful assembly and to disperse from the area immediately," Tucson police chief Roberto Villaseñor said in a a statement. "The majority of the crowd did not respond to the dispersal order, and began throwing beer bottles, beer cans and firecrackers at the officers."

The officers then used pepper ball rounds, pepper spray and foam batons to disperse the crowd, according to the police statement. They arrested 15 people, all of whom are charged with misdemeanors such as unlawful assembly, resisting arrest and disorderly conduct.