Police in Iran say they have arrested 20 young people at a party in the city of Karaj, north-west of the capital Tehran, on Wednesday night.
More than 200 people were arrested a week ago in the same city for attending an illegal rock concert.
Parties, attended by members of both sexes, and where alcohol is consumed, are illegal in Iran.
The Iranian authorities have in recent months increased their enforcement of what is deemed to be moral behaviour.
Dress codes for women have been a particular target.
Police Col Majid Bazmun told the state Irna news agency that police surrounded the building in Karaj where the "decadent gathering" was taking place after acting on a tip-off from a member of the public.
Iran's chief of police, Esmaeel Ahmadi Moghaddam, said that the crackdown of recent months, described officially as the drive to "elevate security in society", would continue as it had proved popular with the public.