A driver for the popular car-hailing service Uber has been charged with seriously injuring a passenger by hitting him over the head with a hammer, prosecutors said Friday.

Patrick Karajah, 26, allegedly lashed out at one of three passengers he had transported late Tuesday in San Francisco, said a spokesman for the city's prosecutors office.

The victim was seriously injured and treated in hospital. Karajah pleaded not guilty Thursday and was released in lieu of $125,000 bail.

"We take reports like this seriously and are treating the matter with the utmost urgency and care," Uber spokeswoman Eva Behrend told the San Francisco Chronicle.

"It is also our policy to immediately suspend a driver's account following any serious allegations, which we have done. We stand ready to assist authorities in any investigation.

"Safety is Uber's No. 1 priority."

Uber is expanding at a rapid clip and now operates in more than 100 US cities. It has a foothold in 42 countries, with a strong presence in the Asia-Pacific region.

But its move into Europe has encountered opposition from taxi companies, including protests from London cabbies and legal injunctions in France and Spain.

In Germany, it was briefly banned, before a court this month threw out an injunction against the service, which lets passengers summon cars using a smartphone app at prices that significantly undercut rival taxi companies.