Despite a threat from the Philadelphia Parking Authority to impound cars and fine drivers, the low-cost UberX ride-sharing service began operating in the city Friday night.

UberX, which runs in 110 cities across the country, is offering service free in Philadelphia this weekend "up to three trips, up to $20 each," said Uber Technologies spokesman Taylor Bennett.

By midafternoon Saturday, the Uber app was showing no UberXs available for Philadelphia.

"Demand is the through the roof," Bennett said.

The Philadelphia Parking Authority took to social media in an attempt to dissuade UberX's target audience from using the service.

"Public needs to know UberX drivers may not be licensed and cars may not be insured. Criminal background checks also in question," @PhilaParking tweeted.

In response, @Uber-Philly tweeted: "@PhilaParking FALSE. Uber's background check is more stringent than yours; insurance is 28x better."

@PhilaParking jabbed: "Legal cabs utilize apps for hailing and payment."

Dennis Weldon, the authority's general counsel, said, "After months of unsuccessfully trying to legalize network transportation services in Harrisburg, Uber's recent announcement that it would nevertheless knowingly provide illegal taxi service in Philadelphia represents a shocking disregard for the law and public safety."

"The PPA will continue to monitor taxi service, and illegal operators will be subject to immediate impoundment and fines," Weldon said.

He said any vehicle providing illegal taxi service via UberX or any other illegal transportation network would be seized, and the driver, owner of the vehicle, and company would each be fined $1,000.

As of 10:30 p.m. Saturday, the Parking Authority had imposed those penalties on five UberX cars, authority spokesman Martin O'Rourke said.

"The public should be warned," Weldon said. "We strongly urge these companies to follow the law, and ask the public to consider its own safety when engaging illegal taxicab service."

Bennett said UberX decided to start service this weekend after last week's liquidation of the First Keystone Risk Retention Group Inc., which had insured more than 25 percent of the city's 1,600 taxis.

Each Uber driver, Bennett said, is backed by the company's $1 million insurance policy. The decision to launch service was to ensure city riders "get safe, affordable rides."

But O'Rourke again emphasized on Saturday that PPA would not take the affected taxis off the streets until Tuesday or Wednesday if they did not have new insurance.

Many taxi owners insist insurance covers them for 30 more days.

Enforcing the ban may be difficult because privately owned UberX cars cannot be identified easily. O'Rourke declined to disclose details of how enforcement would be handled but said the authority would follow procedures previously used to identify illegal taxis.

Uber Technologies Inc. is based in San Francisco. UberX is the company's ride-sharing option, "the low-cost offering where partners use their own cars to transport riders," Bennett said.

Other options include UberBlack, UberSUV, and UberTaxi. The first two services, which use licensed limousine drivers, do operate in Philadelphia, but only after meeting Parking Authority requirements applied to all limos.

Asked Saturday whether any UberX drivers had yet faced action by the city, Bennett replied, "our partners [drivers] are excited for the new opportunity to start moving people around town safely."

aheavens@phillynews.com

215-854-2472 @alheavens