UBERX, that so very 21st-century, app-driven mode of ride sharing that's seeking a Philadelphia foothold in the face of fierce opposition from traditional cabbies and from government officials, hit a giant roadblock on Saturday night.

And its obstacles included one mode of transportation straight out of the 19th century: horses.

Photos posted yesterday on BillyPenn.com showed two mounted city officers on horseback assisting an undercover sting operation by the Philadelphia Parking Authority outside the Ritz 5 movie theater in Society Hill, as two PPA cars surrounded a white vehicle, impounded the car and apparently handed the driver a $1,000 ticket.

A PPA spokesman, Martin O'Rourke, confirmed to the Daily News last night that its sting operation this weekend snarled five drivers for UberX, impounding their cars even as the nationally popular ride-sharing service was hailing its arrival in Philadelphia - prompted by the cancellation on Friday of insurance for about a quarter of the city's conventional taxi fleet - with free rides this weekend for its new customers.

"UberX is illegal - it's an illegal hack cab service," O'Rourke said. He said UberX should have known that its operations were unlawful and expected the crackdown because it had been lobbying state lawmakers for measures to permit its operation here - then went ahead and started up anyway, without that legislative OK.

Before this weekend, Philadelphians had already been able to use Uber, a higher-priced service that only uses licensed limousine drivers. UberX, however, is a related service that now operates in 110 cities and employs regularly licensed drivers in their personal cars, and offers rates at what it claims are 20 percent less than what you'd pay for a taxi in Philadelphia.

UberX and the PPA, which regulates the city's parking service and taxicabs, are locked in a squabble over whether the new would-be ride-sharing drivers have the proper levels of liability insurance and have gone through a sufficient background check.

In a written statement released this weekend to several media outlets, Uber Technologies spokesman Taylor Bennett said: "This is an abuse of power and a deliberate attempt to protect the status quo that has failed Philadelphians for too long. With $1M insurance policy and the most stringent background checks, Uber is the safest ride on the road. Harassing our partners for getting people around town safely when a third of the taxis in the city lack adequate insurance is harmful and irresponsible."

Vince Fenerty, the executive director of PPA, said the exact opposite in his statement. He called UberX "a smartphone app company that connects unsuspecting riders with illegal hack taxis for a fee," and called its Philadelphia operation "a shocking disregard of the law and a threat to public safety."