A witness said Sunday that police raided the concert as it was ending late Wednesday near the town of Karaj, some 30 miles west of the capital.
"Police detained the young people who had gathered to enjoy music in a private orchard," said the witness, who requested anonymity for fear of government retaliation.
The arrests come during a recent crack down on "immoral behavior" in Iran, where holding mixed parties or concerts without permission has been forbidden since the 1979 Revolution that brought hardline Shiite Muslim clerics to power.
Boys and girls mingled and danced together during the concert, and some of the women were not wearing the modest clothing and Islamic headscarf required by law, media reports said.
Dear annoying sheeple cow-orker: Today is not a good day to tell me how the fascist Bushitler regime has stolen all of our civil liberties and turned America into a totalitarian state. But please, do your schtick about how Khatami and Achmadinejad turn into Madison and Jefferson after tea with Barry Obamarama--I could use a good laugh today.