Three men from Pakistan aged between 28 and 31 have been arrested and police are still searching for three men who may also have been involved.
The attacks are similar to those reported in Cologne and other cities on New Year's Eve, when as many as 1,000 women were groped and robbed.
Police said three women reported being groped at the Schlossgrabenfest music festival in Darmstadt on Saturdaym saying they had been encircled then sexually harassed by a group of men.
By Tuesday, 15 more women had made complaints of being sexually assaulted in a similar manner at the festival.