The streets of San Francisco can be a difficult place to catch a taxi, but authorities are doing their best to discourage rogue drivers in unlicensed cabs who can put passengers at risk.

Its just past midnight on a Saturday night in the City's Marina District and the party is just getting started. The pulsing night life scene creates fierce competition for cabs.

KTVU recently got exclusive access to a late-night special operation by the San Francisco Police Department with four officers going undercover to catch the illegal cabs trolling city streets.

"The reason we conduct this operation is because of the complaints we get," said the department's Sgt. Brian Devlin.

It took less than a minute for the decoys to hail gypsy cars for hire. The driver behind the wheel of his sleek black Mercedes is only supposed to pick up pre-arranged customers. Town car and limo drivers are not allowed to pick up passengers on the street either.

But all night long, we watched as shiny town cars and limos pulled over when the undercover officers put an arm out to hail a car. They are just a few of the increasing number of unregulated rides in the city. These drivers skirt the rules to make a quick buck.

A South Bay woman who wished to remain anonymous for this report (we'll call her "Julia") knows why the rules matter. After a night out in San Francisco in 2011, she and three friends tried to hail a cab. The driver of a Lincoln town car pulled up and agreed to give them a ride to the peninsula for $40.

Julia and her friends hopped in the car and headed home. Once they got to their destination, Julia handed the driver a $100-dollar bill. He gave her $20 in change. When she asked for more, the driver snapped and threw the car into reverse.

"The next thing he did was grab my purse. He like ripped it from my hand. He just threw it," said Julia. The driver then grabbed Julia's arms and threw her to the ground. She told the jury at the driver's trial that he repeatedly kicked her as hard as he could, describing it like a punt. He then threatened to run her over.

"Everything was happening so fast. The only thing I could think to do was scream," said Julia.

The jury convicted Khaled Hamoui of kidnapping, making threats and assault. He was sentenced to more than five years.

But other criminal cabbies continue to strike. KTVU has learned San Francisco Police are looking for at least two drivers who recently sexually assaulted their female passengers after picking them up in the Mission. One woman was driven to a dark freeway overpass and assaulted. The other was taken to a desolate alley.

Both cases involve phony cabs, cars designed to look like legitimate taxis but are not licensed by the city. Its these impersonators are the most dangerous, according to Devlin.

"You don't know the driver's history. You don't know his criminal history. You don't know if he's under the influence. You don't know if he's a wanted felon," cautioned Sgt. Devlin. Those unknowns pose a real threat to public safety.

In addition to those busted in the most recent undercover operation, similar stings last year led to 872 citations. But at $200 dollars a citation, it's not always a deterrent.

Investigators continue to chip away at this transportation iceberg sending a message to illegal drivers that it just might be a police officer awaiting them at their next stop.