UFO enthusiasts drawn by alien-themed festivities poured into rural Nevada on Friday (September 20) near the Area 51 US military base, but fears of a mass raid on the remote site or a public safety crisis proved unfounded, with only five people arrested.

More than 3000 out-of-towners ventured on Thursday and Friday to the desert region around Area 51, a secretive US Air Force installation long rumoured to house government secrets about extraterrestrial life and spaceships.

The pilgrimage and two festivals welcoming the visitors were organised after a Facebook user jokingly exhorted readers to "storm" Area 51 on September 20 "to see them aliens." The date chosen for the gathering was never explained.

Lincoln County Sheriff Kerry Lee said the region's guests were largely well-behaved, with just a few exceptions. He also said the numbers of visitors were less than had been predicted.

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"So, it is actually going pretty well, pretty smooth. The numbers are lower than we expected which is good for us because it has not exhausted our resources beyond our capabilities so that has been good but we are thinking roughly 3000 people maybe in Rachel," Sheriff Lee told Reuters on the side of the so-called 'Extraterrestrial Highway' that runs near Area 51.

"So we had some [arrests] overnight and then today we had two additional male suspects that were arrested in the mountains surrounding the Tikaboo/Alamo gate [to Area 51]. They were arrested about 3:30 am today. Both have been transported and booked at our booking facility. We also had another female arrested at about five o'clock at the Rachel gate [to Area 51] and those all three for trespassing," he added.

Three people were arrested on suspicion of trespassing onto the grounds of Area 51, Lee said. Another was arrested for indecent exposure after urinating near an entrance to the base, and a fifth for an alcohol-related incident, Lee said.

A steady stream of cars continued rolling into the county on Friday for events that were scheduled to run through the weekend.

The arrests were reported after a group of about 150 UFO fans and curiosity-seekers gathered outside the heavily guarded entrance to Area 51 around 3am on Friday, the date and time set by the social-media invitation that went viral.