HONOLULU -- The Honolulu Police Department has opened a criminal investigation after seeing video of an officer apparently beating a man who ran onto the Aloha Stadium field after the University of Hawaii football team's win over Washington.

Fans said they were shocked to see what they considered a case of police brutality.

The police Rapid Response Unit was deployed at Aloha Stadium in full riot gear Saturday night. Some fans said at least one of them badly overreacted to exuberant students with brutality and pepper spray.

The interception that saved the game for UH was followed by a small stampede of fans, leaving three seconds left on the game clock. Several fans scooted past on-duty officers in riot gear, but one young man did not make it.

"(The officer) hooks the guy around the head and brings him down," UH freshman Miles Kreisberg said.

Kreisberg zoomed in his camera to the area of the incident.

"I was like, 'Oh, I got to catch this.' I couldn't believe what was going on," Kreisberg said.

The video caught the officer delivering what looked like three quick blows to the fan's back.

"He was just taken down, pinned to the ground, punched three times, four times really hard in the stomach," Kreisberg said.

The fan was shoved down and pushed toward the sideline as the game time expired. The video caught nearby fans reacting to the police.

"People were outraged and shouting 'Stop. What are you doing? Stop! Stop! Stop! Stop!'" UH junior Megan Chan said.

The officers released the fan without officially arresting him, although he was interviewed later. After seeing the video on YouTube, they decided a criminal investigation of the officer was warranted. They are also investigating reports of pepper spray being used on other fans.

"There was a stair entrance where people were running down. Cops were kind of up around face level and spraying things," UH junior Annie Dowling said.

"Both of us were crying. It was really emotional. It completely killed the spirit in the win of the game," Chan said.

We do not know the name of the officer or the fan, who was apparently not seriously injured. The students who were offended have put up a Web site on Facebook to protest called UH Fans Against Police Brutality.