Uber expanded its self-driving car service to San Francisco on Wednesday, but state regulators are calling for a halt. (Courtesy of Uber)

Not even a full day after Uber launched its self-driving service in San Francisco did California regulators tell the company to shut it down.

The California Department of Motor Vehicles threatened legal action, telling the company that it must first obtain a special permit to test autonomous vehicles on the state’s roadways.

Uber launched the service Wednesday morning and acknowledged at the time that it might run afoul of state regulators. The company had declined to obtain a permit on the grounds that its cars require human monitoring and thus do not meet the state’s definition of an autonomous vehicle, a spokeswoman said early Wednesday.

The DMV disagreed. It called Uber’s program illegal and demanded the company cease operations until it received a permit that would require the company to prove that it is financially responsible, has qualified drivers, and will report collisions and other safety information to state regulators.

“These requirements serve to build public trust in the safety of the technology and to foster confidence in allowing autonomous vehicles on public streets,” Brian Soublet, the department’s deputy director and chief counsel, wrote in a letter.

A spokeswoman for the company did not immediately respond to a request for comment on the letter. Uber said earlier Wednesday that it hoped California would see the merits of its self-driving vehicle program, including the potential to improve traffic safety, and not create barriers to innovation.

“Pittsburgh, Arizona, Nevada and Florida in particular have been leaders in this way, and by doing so have made clear that they are pro technology. Our hope is that California, our home state and a leader in much of the world’s dynamism, will take a similar view,” Anthony Levandowski, the head of Uber’s advanced technology group, wrote on the blog.

Uber began matching a small number of riders with vehicles that rely on Uber’s self-driving technology rather than human drivers on Wednesday morning. Those vehicles still had a safety driver on board who can take control if necessary, as well as a company engineer.

San Francisco is the second city in the country to officially test Uber’s service, which the company has said will be critical to the future of both ride hailing and urban transportation. Uber first deployed self-driving vehicles in Pittsburgh three months ago.

“With its challenging roads and often varied weather, Pittsburgh provided a wide array of experiences. San Francisco comes with its own nuances including more bikes on the road, high traffic density and narrow lanes,” Levandowski wrote on the blog.

The rollout in San Francisco started with just a handful of self-driving vehicles, and was expected to gradually scale up as more cars become available, a spokeswoman said. Uber has a partnership with Volvo to provide the company’s self-driving vehicles, including the newly released XC90, which comes equipped with a system of lasers and cameras for guidance.

Only users with a credit card tied to a San Francisco address are eligible for the program. Those who are matched with a self-driving car will receive an alert that allows them to learn more about the program or opt out, a spokeswoman said.

Uber is the second Silicon Valley company to make self-driving car news this week. Google announced Tuesday that its self-driving car project would be spun off into a separate company, called Waymo. The move is a sign that the company intends to bring the technology to market, though an exact timeline remains unclear.

Read more from The Washington Post’s Innovations section

Google is one step closer to making money off self-driving cars

This Honda concept car will have its own emotions