SAN FRANCISCO — Uber is riding its self-driving cars into a legal showdown with California regulators.

The ride-hailing company is refusing to obey demands by the state’s Department of Motor Vehicles that it immediately stop picking up San Francisco passengers in self-driving cars.

Hours after Uber launched a self-driving service Wednesday morning with a handful of Volvo luxury SUVs, the DMV sent the company a letter saying the move was illegal because the cars did not have a special permit the department requires for putting autonomous vehicles on public roads.

As of Wednesday night, the Volvos  were still roaming San Francisco’s streets. The company did not respond to a request for comment about the legal threat.