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Last week, when I arrived at the Los Angeles airport on a flight from San Francisco, I made my way to the taxi stand and waited 10 minutes for a cab. Just as I was about to hop in, the driver and the dispatcher began fighting over whose job it was to put my suitcase in the trunk. After a few minutes, I dealt with the bag myself. We then drove off in a filthy taxi that smelled like cigarette smoke and had suspension so old that it felt as if it had square wheels.

This made me think once again: the taxi industry is ripe for disruption.

Several companies in Silicon Valley — like Uber, Lyft and Sidecar — are acting on that very thought.

Uber, like the other services, does not have its own fleet of cars. The company teams with existing luxury car services and acts as a digital dispatcher for people booking a car through its mobile app.

Although Uber determines price the same way taxis do, calculating fares by time and distance, the service can cost 50 percent more than a normal city cab. Another service offered by Uber, called UberX, offers passengers a lower-cost ride in hybrid cars and is comparably priced to traditional taxis.

But companies like Uber are continually confronting the obstacle of entrenched government bureaucracy, resistant unions of taxi drivers and dispatchers, and overlapping and sometimes conflicting systems of state and city regulation.

The latest roadblocks have come up in Los Angeles, where Uber began offering its service in March.

Travis Kalanick, the company’s co-founder and chief executive, said it received a cease-and-desist letter from the city of Los Angeles, “even though this is not under their jurisdiction.” He added, “The taxi industry feels it is getting disrupted and they are doing whatever they can with their lobbying relationships to try and stop us.”

The letter, issued by the Los Angeles Transportation Department, says that Uber is “operating an unlicensed commercial transportation service” in the city. Jonathan Hui, an agency spokesman, wrote in an e-mail that it was working with the mayor’s office “to determine ways to address ride-sharing companies,” but he declined to comment further.

William Rouse, general manager of Yellow Cab in Los Angeles, one of the largest taxi companies in the city, and president of the Taxicab, Limousine and Paratransit Association, a trade group, said that making sure taxis were officially licensed was a matter of public safety.

“Our roadways are a scarce resource,” he said. “When you have an oversupply of taxi cabs, you have more congestion, depressed driver incomes and poor service. It’s also an issue of public safety, where drivers aren’t insured and there are felons driving some of these ride-sharing apps’ cars.”

Uber and other ride-sharing apps say that all their cars are insured over the required taxi insurance limits and that drivers must undergo stringent background checks.

“This isn’t about safety,” said John Zimmer, co-founder of Lyft, which also received a cease-and-desist letter from Los Angeles regulators. Mr. Zimmer said he believed the real opposition from taxi companies was not about customer safety, but fear of competition.

Mr. Zimmer also pointed out that state regulators had approved his service.

The California Public Utilities Commission, he said, lets it operate “because we go above and beyond all of their guidelines.”

Uber has been dragged through regulatory hurdles in New York City since its introduction there in 2011. But last month Uber had a victory of sorts, winning a ruling allowing it to operate in Cambridge, Mass. This came after city officials, at the behest of local taxi companies, tried to ban Uber even though state regulators had already approved the operations of such ride-sharing services statewide.

The Federal Trade Commission has also recently issued a statement supporting the services; the agency said trying to snuff out ride-sharing apps would stifle competition and could hurt consumers.

The controversy in Los Angeles is par for the course for start-ups that have come up against regulators. They have been sued or received cease-and-desist letters from almost every city they operate in.

When it comes to protecting customers on pricing and overcharging, taxi regulation makes sense. But in some instances, regulatory bodies have done more harm than good. In 2009, for example, more than 30 people in Washington, including at least one city official, were indicted on bribery charges during talks of regulatory change.

Some lobbying groups, meanwhile, are using fear tactics. In March, Mr. Rouse’s taxi industry group issued a news release warning that companies like Uber and Lyft were “rogue transportation apps” and a “threat to public safety.” The release said that arguments involving payment “could turn violent,” but offered no examples.

Mr. Kalanick of Uber said cities should simply let the customer decide. “The taxi groups are so protected through these regulations that they do not have to offer a better service to customers.”

Mr. Rouse acknowledged that cab companies needed to do a better job, and said he was working with cabdrivers to increase quality of service. But he said taxi groups “will continue to advocate for law enforcement against these ride-sharing apps.”

Although the services do not share figures on how popular they are, the opposition suggests they are posing a real threat. As for me, I’m glad when I have a choice.

To get to the San Francisco airport, before my recent flight to Los Angeles, I called an Uber car. When the driver picked me up, he was nervous about taking me to the airport, he said, because its police were ticketing Uber drivers. The service is allowed in San Francisco, but the airport police follow different rules.

“I can take you,” the driver earnestly said. “But we will have to pretend to be related so I don’t get a ticket.”

When we pulled up to the airport, he got out of the car with great ceremony, handed me my bags and followed his script. “Have a safe flight!” he said, giving me a big hug while peering sideways for signs of the law. “I’ll see you soon.”

E-mail: bilton@nytimes.com