UFO enthusiasts are descending on rural Nevada near the secret US military installation known as Area 51, long rumoured to house government secrets about alien life.

Early on Friday (Australian time), dozens of visitors began arriving at the only business the tiny desert town of Rachel, Nevada, near the military base – an extraterrestrial-themed motel and restaurant called the Little A’Le’Inn – in response to a viral social-media invitation.

Some of Rachel’s 50 residents are worried their community might be overwhelmed by unruly crowds turning out after more than two million people signed up to “Storm Area 51, They Can’t Stop All of Us”.

The joke event, posted to Facebook by California college student Matty Roberts, is planned for September 20,

Mr Roberts’ “rationale” for his June invitation was simple – if enough people stormed the gates of the secretive base in the Nevada desert, there would not be enough guards to stop everyone from getting in. Their goal?

“Let’s see them aliens,” the post said.

Area 51 has been shrouded in secrecy for decades, stoking conspiracy theories that it housed the remnants of a flying saucer and the bodies of an alien crew from the crash of an unidentified flying object in Roswell, New Mexico, in 1947.

The US government would not even confirm the base existed until 2013, when it released CIA archives saying the site was used to test top-secret spy planes.

When the interest in the Facebook event became overwhelming, the US Air Force warned curiosity seekers not to breach the gates at the military base, which it said was still used to test combat aircraft and train personnel.

But still the interest grew. Then Mr Roberts teamed up with the Little A’Le’Inn to plan a music festival, dubbed Alienstock, in Rachel to entertain the expected crowds.

Area 51 doesn’t exactly lay out the welcome mat for visitors.

He’s since disassociated himself from that event, saying he fears it might devolve into a public safety crisis because of the sheer weight of numbers anticipated to attend.

“My entire thought process for creating this was that it was going to be something funny for my meme page that probably a maximum of 52 people would see,” he told the ABC.

“I was absolutely terrified [when the FBI called]. But they were just concerned for my safety and the safety of others. They were doing their due diligence so I can’t be mad about it.”

The official website for Rachel, Nevada, echoed his warnings with a strident warning to intending visitors. It bold red font, it states:

“We expect riots when those visitors that may show up and paid good money find out that the reality looks nothing like what they were promised. People will get hurt.

“STAY AWAY FROM RACHEL NEXT WEEKEND!”

Mr Roberts is now promoting a new “safer alternative” planned for a more than two-hour drive from Area 51 in downtown Las Vegas. It even has a brand deal by way of a specially-created Bud Light beer can for the event.

There are 11,500 RSVPs, with another 16,000 trying to get in, according to Mr Roberts’ event planner Frank DiMaggio. He expects a capacity crowd.

#StormArea51: Preps continue day before Area 51 Basecamp event https://t.co/jVZ75LJyZy #8NN

— 8 News Now (@8NewsNow) September 20, 2019

Back in Rachel, the Little A’Le’Inn’s co-owner Connie West said the local event would go on as planned – despite the fears of possible chaos.

“It’s happening. There was no stopping it,” she said, adding that some visitors had come from as far away as Australia and New Zealand.

“I hope they just enjoy the party we are throwing.”

Earlier, Lincoln County Sheriff Kerry Lee told the BBC that an extra 150 officers and 300 paramedics were being brought in to help deal with this weekend’s expected crowds.

An emergency declaration has been presigned, just in case.

There are real concerns among locals about an influx of people to a place that has very limited services.

Chairman of the Nye County Commission Jon Koenig, which is nearby, predicted there will be no mobile phone service, because the area’s cellular towers aren’t built for the volume of calls.

“There will probably be no water … or ice available, because everything’s going to sell out,” he said during a Wednesday public meeting, the ABC reports.

“There will be no gas left in the gas stations. No food. Nowhere to go potty.”

America’s Federal Aviation Administration has also issued a flight ban for the entire area until Monday (local time), meaning everything from news helicopters to drones aren’t allowed to fly.

Even police and medical helicopters will need to get special permission to fly, if required.

-with agencies