WASHINGTON — Summoning a taxi or car service with your smartphone feels like the future. City governments around the world can agree on that. But many of them are proposing new rules that would run Uber, one of the most prominent ride-requesting apps, off the road.

At a recent conference here, transportation regulators and car service operators from cities in the United States and Europe met to talk about how smartphone apps were changing the hire-a-car business. Some of these apps are integrated with dispatching systems run by the car companies, while others allow drivers to directly connect with passengers, phone to phone.

While the regulators discussed ways to clarify the legality of these apps, they also proposed guidelines that would effectively force Uber, a San Francisco start-up, to cease operations in the United States. Uber also faces new lawsuits filed by San Francisco cabdrivers and Chicago car service companies, and a $20,000 fine from the California Public Utilities Commission.

The battle between Uber and city governments underscores the tension between lawmakers and technology companies at a time when Web sites and mobile apps can outmaneuver old rules. Services like Uber, Airbnb and Craigslist can cut out the middleman and lead to more efficient markets. But regulators say they could also put consumers at risk.