The scene: a Saturday afternoon in December. A Santa appears in the frame, walking through Old City. Muscular and tall, not your usual Santa, but, OK, you can deal with a slender Santa.

Then a short, blond guy Santa appears in Rittenhouse Square. Then a goateed African American Santa, similarly appointed in the usual red-and-white fur, materializes in Washington Square. Two redheaded women, decked in Santa gear, appear in University City and the Italian Market.

That's when you realize: You're in the middle of the Running of the Santas, a Philly-originated tradition (created 1998) in which Kris Kringle-suited men and women barhop in the name of holiday revelry, good fellowship, and a daylong, boozy buzz. Running of the Santas has become a worldwide phenomenon, with 20-plus host cities running with white beards and high-buckle boots.

In 2013, the Philadelphia event has expanded into a 15-hour Running of the Santas Mega Festival. About 10,000 expected participants will go on walkabout throughout the Spring Garden area Saturday, starting at Finnegan's Wake, before settling into an all-day music showcase at the Electric Factory.

"It's kind of like a Hunter Thompson hallucination," says Michael McNally, general manager of the Electric Factory, "watching 5,000 free-flowing Santas coming towards you in a sprint to the finish."

All this is a far cry from the event's origins, during the warm summer of 1998. Two Jerseyites - Matthew McDermott of Haddon Heights and Ryan Van Laeys of Washington Township - met while working at the Ocean Drive Bar in Sea Isle City. They hatched this wintry idea, and that Christmas season they gathered 38 of their friends to pub-crawl through Manayunk - dressed, of course, as Santas. Who wouldn't?

"A winter festival was never really attempted before Running of the Santas," says McDermott, "let alone a costumed one, and we felt like it was a no-brainer, so we did it. Have you seen how excited the city gets for the Mummers Parade? Philly loves this stuff."

Finding 40 Santa suits for the first run was surprisingly easy. "We put those suits in the trunks of our cars to hold until the next year," McDermott says, "but after the third year, our suits started to decompose. Then, in the fourth year, I went to a party supply store, Party City, and the Santa suits were sold out. The cashier told me there was a Running of the Santas event that weekend, and 'the Santa suits have been flying off the shelves.' " That's how McDermott knew the run had arrived.

After several years in Manayunk, McDermott's Santas moved their liquored-up stroll to South Street and became bigger because of it - 1,000 Santas by 2005. In 2006, the event moved to Northern Liberties and teamed up with McFadden's on Third Street to create a winter festival to follow the pub crawl. By 2009, the crazy bash had outgrown McFadden's and moved to the Electric Factory. "We're growing every year," McDermott says.

It's not totally a bad thing that shops run out of Santa suits. The dearth spurs creativity. Many participants are goaded to transcend the familiar red-and-white furry thing. Pub crawlers don more over-the-top outfits. Think: reindeer, Christmas trees, elves, and even Jewish seasonal toys such as dreidels. "One year," McDermott says, "a guy came in street clothes holding a real Christmas tree."

Joseph String of Deptford has been participating in the Running of the Santas since its 1999 second crawl. He makes certain he has never worn the same costume twice. "I've been a 'traditional' Santa, a tuxedoed Santa, a toy soldier, to one of the three wise men," he says. "It helps you get your mind off of everything, from the necessities of holiday shopping to the bills you'll pay after Christmas."

McDermott, who trademarked his event to maximize his brand's potential, says most of the money from Running of the Santas (tickets are cheap, $15) goes to marketing expenses in other cities.

Other cities? Yes.

As Philly's Santa run grew, McDermott, 36, found that many local Santas had moved throughout the United States and asked him to help them start Santa runs in their adopted cities. "Another no-brainer," McDermott says - but he will license it only to people who have seen Philly's original event in person.

There are now Santa runs from all over the United States to New Zealand. But, McDermott says, only Atlantic City and New Orleans make as much of a fuss over their runs as Philly does.

"I'll be in Louisiana again this year," he says. "It's a sight to see down there."

Still, you can't beat the Philly original. You have North Poles and South Poles set up for drinking. Famed local rockers such as LeCompt, Jersey cover bands like Go Go Gadjet, and national acts like Tonic will grace the stage at Electric Factory.

The run is a great place, says McDermott, for single Santas to meet: "You may find your future husband or wife. It's happened many times. In 2010, a couple of Santas met and got married during a Running three years later." The bride wore her wedding gown to the Running, and the Phillie Phanatic presented the couple with a wedding cake.

So what does McDermott - with a newborn daughter whose Santa outfits are ready for Saturday - want for Christmas? "Three days of sleep after this weekend would be nice."