At a certain point on Saturday afternoon, the boundaries dissolved between the Irish bars that dot Sansom Street in Center City, and the street fused into a single, multi-block expanse of revelers dressed in green T-shirts, green beads, green plastic bowler hats, green mouse ears, green sunglasses, and tiny, green, paste-on shamrock tattoos.

"Instead of going to Cancun, I'm here on the Erin Express," chortled Khalil Jones, 22, a senior at Rutgers University in New Brunswick, N.J., who decided to use his spring break to visit Philadelphia for its increasingly famous - and increasingly extended - St. Patrick's Day celebrations. "I read about it on and, and wanted to see it for myself."

The Erin Express, an all-day pub crawl sponsored by a group of bars and held on two consecutive weekends in March, hasn't quite reached Mardi Gras proportions, but the St. Patrick's-themed event shares many of its party-hearty characteristics, including intensive day-drinking.

"It's a long day, so you have to pace yourself," explained Juliana Latchun, 23, a Drexel nursing student who was decked out in green nail polish and green reflective sunglasses.

It was her third Erin Express in as many years, so she had worked out a system: Enjoy a drink. Move onto another bar. Eat. Drink a little more.

It used to be, back in the olden days, that March 17 was the only day when people conspicuously showed off their Irish - regardless of whether their families hailed from the Emerald Island - by wearing green, watching the St. Patrick's parade, and hoisting a few pints. But like so many holidays, St. Patrick's Day now starts earlier and continues longer. In Philadelphia, it's become St. Patrick's Month.

The Erin Express is the main reason why. The event has taken the crawling out of the pub crawl.

Some four decades ago, the owners of Cavanaugh's and Smokey Joe's convinced a group of bars to chip in for buses to take people around to different bars. This year, there were a dozen yellow school buses, including two expresses that traveled without a stop from Center City to West Philadelphia, ferrying the mostly twenty-something merry-makers on a 15-bar circuit, said Patrick Pawliczek, Cavanaugh's proprietor.

That Erin Express weekends frequently coincide with spring break at many colleges has only increased its draw. Many come back year after year.

Nick Torsitano, an NYU student whose family lives in Horsham, came home for the event so he could get together with old friends. By 2 p.m., they had already been to three bars, and were waiting on a line at Oh!Shea's Pub on Sansom Street.

"I was in New York City last weekend for their event, but nothing is as crazy as this," Torsitano marveled. "We drank a lot of [whiskey] earlier," he said, "but now we're switching [to light beer]."

Complimentary Red Bulls from a sponsor table were helping the group keep going. Fortunately there were a line of Porta-Potties lined up outside many of the bars.

The extra facilities are a response to complaints from Center City residents who say the Erin Express is turning into an unruly bacchanal. The 2012 event, which actually fell on the real St. Patrick's Day, drew massive crowds. Last year, there were a few brawls, and at least one shop window was broken by a rock-throwing reveler.

"In theory, Erin Express is a great idea, a great way to expose people to a celebratory experience in Center City," said Corie Moskow, the executive director of the Rittenhouse Row merchants group. "But," she added, carefully choosing her words, "it can get out of control. It's a shame when a couple of idiots do stupid things."

This year, there were noticeably more police out, one cruiser at every bar along Sansom Street. There has also been talk of closing the street for the day of the event, to contain the party.

Pawliczek argued that the good of the event outweighs the bad. Critics "see 15 or 20 people in green shirts and think it's some kind of invasion," he said. "The city is trying to draw millennials, so they stay in the city, raise families here, and pay taxes.

"It's truly amazing what a phenomenon it's become."

So much so that Olivia Duca, of Haddon Township, N.J., had been waiting until she turned 21 so she could participate. The nursing student was giddy as she stood with a group of friends waiting for the Erin Express at Headhouse Square to take them to Cavanaugh's in University City.

"We're fresh meat," she joked.