A call to "storm" the secretive US military base in the Nevada desert known as Area 51 attracted several dozen revellers to a heavily guarded entrance early on Friday, but most did not attempt to enter the site, long rumoured to house secrets about extraterrestrial life.

Key points: Many of the revellers dressed in costumes, including tin foil hats

Many of the revellers dressed in costumes, including tin foil hats A large police presence kept a close eye on the small crowd

A large police presence kept a close eye on the small crowd More than two million people signed up for the original "storm Area 51" event, most as a joke

A festive scene emerged around 3:00am local time on Friday, the date and time a Facebook user had invited people to run into the base on foot to "see them aliens".

Among the UFO enthusiasts and curiosity seekers, one man wore an orange space suit and some sported tin foil hats and alien masks.

A sign in the gathering read "Free ET from the government".

"A bunch of random people in weird costumes standing outside of a government base, why would you want to miss that?" a YouTube personality who goes by the name Atozy said.

"That's a once-in-a-lifetime experience."

People holds signs at an entrance to the Nevada Test and Training Range near Area 51. ( AP: John Locher )

One young woman ducked under a protective gate and was briefly detained by authorities and released. Others stayed outside the perimeter, according to law enforcement officials keeping watch over the crowd.

"They're just here to see what's going on," Sergeant Orlando Guerra of the Nevada Department of Public Safety Investigation Division, said.

"They're here to have fun."

The US Air Force had issued a stern warning to the public not to trespass into Area 51, which it said is used to test aircraft and train personnel.

Many of the revellers dressed in costumes, including tin foil hats. ( Reuters: Jim Urqhart )

Facebook post that grabbed public's attention

Millions of people had responded to a June internet post calling for people to run into the remote US Air Force test site that has long been the focus of conspiracy theories about aliens.

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Jason Strand, 23, said he travelled from Utah to the rural Nevada site as part of a group of nine friends to take in the scene, but said he was not inclined to dart into the military base.

"We came out here to see the dumb people make a run for it," he said.

Area 51 has long been shrouded in mystery, stoking conspiracy theories that it housed the remnants of a flying saucer and the bodies of its alien crew from a supposed unidentified flying object that crashed in Roswell, New Mexico, in 1947.

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

The documents, however, did not end suspicion about aliens.

The Area 51 site is located about 19 kilometres from Rachel, Nevada, a tiny outpost north of Las Vegas that is hosting a music festival to entertain any UFO hunters or others heading to the region.

Some residents urged the public to stay away because they worried the town of 50 year-round residents would be overwhelmed by unruly tourists.

As of early Friday, a few hundred campsites had been set up by visitors outside the Little A'Le'Inn, an alien-themed motel and restaurant that is Rachel's only business.

There is plenty of Storm Area 51 merch. ( Reuters: Jim Urquhart )

"I'm relieved it's here," inn co-owner Connie West, who had scrambled to set up a campground, said.

"It's happening. There was no stopping it."

Reuters/AP