About 75 people have arrived at a gate at the top-secret Area 51 military base in Nevada and at least two were held by police.
The Storm Area 51 invitation spawned festivals in the tiny nearby towns of Rachel and Hiko, more than two hours’ drive from Las Vegas.
Millions of people responded to a June internet post calling for people to run into the remote US Air Force test site which has long been the focus of UFO conspiracy theories.
“They can’t stop all of us,” the post joked. “Lets see them aliens.”
“It’s public land,” the sheriff said. “They’re allowed to go to the gate as long as they don’t cross the boundary.”
A music group called Wily Savage erected a stage on Thursday near the Little A’Le’Inn in Rachel and began playing after dark for several hundred campers who braved overnight temperatures of about 7C.
The entertainment kicked off weekend events that also feature a gathering on Friday and Saturday at the Alien Research Centre souvenir store in Hiko.
Authorities reported no serious incidents related to festivals scheduled until Sunday. Hiko and Rachel are about a 45-minute drive apart on a state road dubbed the Extraterrestrial Highway.
Eric Holt, the Lincoln County emergency manager, said he believed authorities could handle 30,000 visitors at the two events, but residents are still braced for trouble.
But those who show up in shorts and flip-flops will find no protection against “critters, snakes and scorpions”.
“It will get cold at night. They’re not going to find what they’re looking for, and they are going to get angry,” Mr Arnu said.