Portland sued Uber on Monday to stop the fast-growing ride-hailing service from operating in the Oregon city until it follows local regulations.
Uber operates in areas around Portland, but only started up in the city itself on Friday, without consent from authorities or any agreement over how it should be regulated.
"The city's lawsuit is asking for a declaration by the court that Uber is subject to the city's regulations," the city said in a statement. "The lawsuit also asks the court to order Uber to stop operating in Portland until it is in compliance with the city's safety, health and consumer protection rules."
Local media reported that the service has been available in Portland for the past few days.
"Uber has received a tremendously warm welcome from riders and drivers in and around Portland. We appreciate the way residents have welcomed Uber into the Rose City, their support illustrates why it's time to modernize Portland transportation regulation," said Uber spokeswoman Eva Behrend.
The online ride service was valued at $40 billion last week after its latest funding round ahead of an expected initial public offering.