At least two people were detained by local sheriff's deputies at a gate to Area 51 in Nevada after an estimated 75 people gathered at the site early Friday.
A weekend gathering is happening in the desert after an internet hoax posted in June invited people to "storm" the once-secret military installation. The place is the focus of popular lore about government studies of extraterrestrial life and space aliens.
It wasn't immediately clear if a woman who began ducking under a gate and a man who urinated nearby were arrested after the crowd gathered about 3 a.m.
An Associated Press photographer saw both escorted away by deputies. A sheriff's dispatcher declined to comment.
Sheriff Kerry Lee was scheduled to speak to the media later in the morning.