In California, after a transit cop and an unruly train passenger slammed against a wall during a struggle and shattered a station window last fall, video from a bystander's cell phone was all over the Internet before the window was fixed.

The same cell phones, surveillance cameras and other video equipment often used to assist police are also catching officers on tape, changing the nature of police work - for better and worse.

Some say cameras are exposing behavior that police have gotten away with for years. But others contend the videos, which often show a snippet of an incident, turn officers into villains simply for doing their jobs, making them targets of lawsuits and discipline from bosses buckling to public pressure.

"We tell our officers all the time you've got to assume that everything you do is going to be videotaped," said Chicago Police Superintendent Jody Weis. "Everyone has a cell phone and almost every cell phone has a camera."

Cook County State's Attorney Anita Alvarez said the video her office gave to the media on Tuesday shows police officer James Mandarino, from the Chicago suburb of Streamwood, hitting motorist Ronald Bell 15 times after a traffic stop last month.

In the video, Mandarino is seen firing a Taser at a passenger in the car and then striking Bell, who is on his knees with his hands on his head. Bell suffered a concussion and cuts that required seven stitches.

"It's a wonderful tool," Alvarez said of the video, which she says suggests that both men posed no threat to the officer.

Though police-behaving-badly videos have become popular staples of cable news shows and the Internet, Weis said he doesn't believe his officers are overly cautious out of fears they'll be videotaped - and their superiors are not advising them to be.

Quietly, though, some officers say the prospect of being videotaped makes them hesitate even if they know they should act.

"I've heard from officers who are sent to break up a fight in the street and see a group of people leaning out windows with handheld video cameras ... they go slower and are less aggressive," said Tom Needham, a Chicago attorney who has represented several police officers.

But University of Chicago law professor Craig Futterman, who has studied police brutality, said videos are helping hold police accountable.

"My own view is that YouTube has done more to expose the reality of police abuse than all the blue-ribbon commissions combined," said Futterman.

A Chicago police officer who was arrested three years ago in the videotaped beating of a female bartender never would have been charged much less convicted if not for the video, Futterman said. Anthony Abbate initially was charged with a misdemeanor until the video played across the world.

Ronald Bell's brother, Stacey Bell, said he doubts the Streamwood officer would have been charged with felony aggravated battery and official misconduct without the video and his brother still would have faced charges of drunken driving and resisting an officer, which were dropped.

"I believe it would have been six witnesses against an officer and it would have been a different story," said Stacey Bell, who witnessed the alleged beating. The officer's attorney declined to comment.