Radan said the group had been planning the water fight through the Internet and had "intended to break customs." He vowed that police would act to prevent future attempts.
Throughout the summer, Iranian police have been cracking down. In the first incident, in July, hundreds of young men and women held a water fight in Tehran's popular Water and Fire Park, spraying each other with water guns and splattering bottles of water on one another. Police detained dozens of those involved.
Since then, police have arrested dozens more involved in similar water fights in parks in major cities around the country.
Hard-liners see the water fights as unseemly and immoral, breaking taboos against men and women simply mixing, much less dousing each other with water and playing in the streets.
But authorities see a darker hand as well, worrying that the gatherings could weaken adherence among young people to Iran's cleric-led Islamic rule or even build into outright protests against Iran's regime. Iran's leadership has been wary of any gathering, whatever its nature, since the massive protests against the 2009 re-election of President Mahmoud Ahmadinejad.
The anti-regime uprisings that spread around the Arab world this year only add to the leadership's worries of any sign of "people power."
Yesterday, the spokesman of the judiciary, Gholam Hossein Mohseni Ejehi, accused unnamed foreign hands of organizing the water-gun campaign.
"This is not simply a game with water. This act is being guided from abroad," he said. Some of those detained Friday have admitted "they were deceived, and some said they came out based on a call from a counterrevolutionary," he said, quoted in the conservative news website Tabnak.
State TV has aired statements by some arrested in previous water-fight crackdowns, admitting they had been motivated by "foreign invitations." Some confessed that they had been given water guns to use. Most detainees were released afterward.
Many of the water fights are organized through calls on Facebook, which is banned in Iran, though Iranians frequently access it through proxies. Most of the Facebook pages are not expressly political, but they express the sort of secular youth culture of Iranians unhappy with the country's Islamic rule.
Iran frequently accuses the United States and Iranian opposition groups in exile of fomenting opposition activity on its soil.
Friday's water fight had been planned to be held in Tehran's Water and Fire Park, named for its numerous fountains and light shows.