When thousands descended upon Seaside Park for a meet-up organizers promised would be "fire," officials were surprised.

And then they were overwhelmed.

The meet-up, spread as a flier on social media accounts without any notice to officials, left police and first responders scrambling to account for the chaos of an estimated 15,000 additional beach-goers (yes, that's on top of the usual crowds for a busy summer Saturday).

Some stormed past a single officer along the boardwalk, skirting beach fees, and a handful ended up in brawls caught on camera, which eventually elevated the underground meet-up to the national news stage.

Warning: The video below contains violent content and graphic language.

Officials said there was no DJ as promised, and no evidence of an organized event -- aside from the thousands of expectant people.

"Somebody just went on social media and posted this. It attracted the people that came down," said Cpl. Stephen Shadiack of the Seaside Park police. "When they got here, they found out there was no such thing. All fake."

Attendees didn't seem to mind. Hundreds of posts were made under the event's hashtag of pictures of groups of young people smiling on the sand. An account setup under the meet-up's name, "#Beachniknj18," has more than 1,000 followers.

In fact, the meet-up was so wildly successful, many are already talking about holding another later this summer.

A message sent to the Instagram account was not returned Tuesday.

That prospect has police scouring hashtags on Instagram to learn more about the new culture of organizing, trying to stay ahead of a second iteration. Other than that, it's not clear what, if anything, they can do to put a stop to the masses.

"It's something that we have to talk about," said Seaside Heights Mayor Anthony Vaz. "It's concerning because it's never happened before."

The municipalities require organizers to obtain permits to host events like the one advertised, with DJs or loud music. Often, the planners are on the line for covering police overtime pay, trash removal and any expenses associated with shutting down a beach.

But when the event is a pop-up, drawing people to public spaces without altering or closing them, the line discerning who's responsible gets blurred. And when the event quickly boils over to a public safety concern, it falls immediately to police, who know little to nothing about it.

They can look for open containers, or haul off those involved in fights. But for the general crowds that often facilitate those acts, there's little to do but watch and wait.

On Saturday, it was local and county-wide officials who stepped in to help the Seaside Park Police Department with crowd control, that had to take on responsibility.

"We had to scramble to get other ambulances there," Seaside Park Chief Francis Larkin said. "It'd be nice if we'd had notice."

While the crowd resembled that of Memorial Day or Fourth of July, when police know they have to add resources to handle an influx of vacationers, it was the lack of notice that left police blind-sided.

Particulary because a first-round of posts a week ahead of the event advertised the event named a beach in Seaside Heights as the location, while a second, posted just a day before the meet-up, said it would take place in Seaside Park -- a location with "less rules and regulations."

At an official event, officers would know ahead of time they were putting in extra hours, and officials would have time to do a threat assessment, looking for any signs of violent activity in large crowds, Larkin said.

Between Saturday and early Sunday morning, Larkin said his department logged 51 calls for service, resulting in two arrests and the issuing of 14 public nuisance summonses, a single citation for having alcohol in a vehicle, and three for sleeping in a motor vehicle.

That number wasn't particularly high for a summer weekend, he noted, but it's likely the large crowds forced officers to concentrate efforts on the meet-up area and that minor offenses elsewhere in the borough went unnoticed.

In neighboring Seaside Heights, which assisted Seaside Park, officials are concerned about similar meet-ups.

Vaz, from Seaside Heights, said it's too soon to speculate if people who start such firestorms on social media could face fines from the borough, or what the best response is. Vaz said he planned to raise the issue during Wednesday's council session, when the borough attorney would also be present to give input.

Larkin said his department would respond similarly if the masses returned; calling into county officials to get mutual aid from other departments. Other than that, and chasing ever-evolving hashtags to keep up, there's not much more police can do but monitor crowds that come largely with plans of behaving.

"Unfortunately, it's okay to do. If 10 of us are going to meet at the beach, why can't 10,000 of us?" he said. "It's amazing, the power of the phone."

Amanda Hoover can be reached at ahoover@njadvancemedia.com. Follow her on Twitter @amandahoovernj. Find NJ.com on Facebook.

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