The "Storm Area 51" viral Facebook event that once jokingly called for people to crash the fabled military base was more calm than "storm" Friday morning - but local authorities in the US state of Nevada remain on alert as tourists continue to stream into the desert for spinoff events throughout the weekend.

About 100 people gathered outside of Rachel, Nevada, starting at 2.30am Friday (local time) - the date and time of the original "Storm Area 51, They Can't Stop All of Us" event, where Nevada public safety officials described the tinfoil-hat-wearing, inflatable-alien-toting crowd as peaceful and compliant. For about three hours, officials said groups lingered at various gates outside the Nevada Testing and Training Range, which encompasses the so-called Area 51 site.

But the relative calm of the morning gave way to more hectic reports as the crowds grew in Rachel and the nearby towns of Alamo and Hiko, Nevada.

• UFO fans ready to 'storm' Area 51, 'see them aliens'
• Only 75 people show to 'storm' secret military base Area 51
• US Navy confirms those UFO videos are the real deal

As of Friday afternoon, at least two people had been arrested since the first revellers started to arrive on Thursday night, Lincoln County Sheriff Kerry Lee told The Washington Post. One person was arrested for public indecency after urinating on a gate, and another was booked after an alcohol-related arrest turned up an outstanding warrant. Lee said a few more people were taken into custody for trespassing and released, and one person was injured in a rollover accident.

"It's really picking up. We're getting hundreds of visitors right now at the Rachel gate," Lee said Friday afternoon.

Eric Holt, the fire chief and emergency manager for Lincoln County, told The Washington Post that about 1500 people had arrived in both Rachel and Hiko on Thursday night. With virtually no accommodations, people were camped out in RVs, cars and tents.

Even though the tourists numbering in the thousands is a far cry from the more than 2 million who responded to the online event, Holt said just 10,000 tourists more than doubles the local population and pushes resources to the brink.

"If we get ten thousand people, it overwhelms our gas stations and grocery stores. The county of ten thousand square miles with a population of about 5000," Holt said.

The town of Rachel, for instance, has just 54 residents. The closest gas station is in Alamo, about 96 kilometres away. Holt worried about out-of-towners unfamiliar with the remoteness of the area.

"They're coming out to the desert thinking Las Vegas is hot, but it gets cold in the desert. And they need to bring extra food, gas and water to support their travel needs," Holt said. "It's a definitely burden on our economy and our emergency resources."

Holt, for instance, had been up at 4am after ending the previous day's shift at 11pm. The neighbouring Nye County Sheriff's Office and other state public safety entities are assisting with the response.

"It's going to be a long week," Holt said.

Later in the day, Lee had a more optimistic view noting that, all things considered, the situation was going well.

"The service stations haven't run out of gas, they brought food trucks in... and I've seen some very interesting costumes. I'm pretty impressed with it."

He said as the number of people at the event sites swells from the hundreds to the thousands, the complaints have remained minor.

"We had some calls at the Little A'Le'Inn," Lee said of an Alamo, Nevada-based inn that is serving as one of the base camps. "But they're squabbles - 'get out of my campsite' stuff."

The event's creator, 21-year-old Matty Roberts, technically pulled the plug on the gathering in early September over fears of a potential "humanitarian disaster".

The Bakersfield, California, college student first created the Facebook event earlier this summer as a harmless joke; at best, maybe it would inspire a few good alien memes. Instead, it mushroomed into a viral spectacle that became a magnet for media attention (and scrutiny from the Air Force).

Roberts then parlayed his joke event into a real-life festival dubbed "Alienstock". He set his sights on the tiny town of Rachel and even found a local business partner in Connie West, the owner of the Little A'Le'Inn. But as interest in the festival grew, so did Roberts's concern that thousands of people would overwhelm a 54-person town with no gas station or grocery store.

"There's no safety or security that can really be promised," Roberts told The Washington Post at the time. "I didn't feel comfortable with inviting even my friends and family out to this event, let alone these thousands of strangers."

With barely two weeks to go, Roberts declared Alienstock in Rachel cancelled, over West's objections. Roberts then announced he was counter-programming a three-day festival - the official Alienstock - at a convention centre in Las Vegas, complete with sponsors such as Bud Light, Arby's and Kool-Aid.

West said she would hold the music and art-filled festival without Roberts's involvement, despite his lawyer sending her a cease-and-desist letter.

Asked Thursday about the road from late-night online joke to corporate sponsored three-day festival, Roberts indicated he had moved past the desert dust-up and plans to ride the alien party wave as long as he can.

"Who could have predicted this?" Roberts said to KTNV 13 in Las Vegas. "You make a post at 2 am on a page that has 62 likes and now you have thousands of people that are ready to come out to your show."