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 the ruling system. Iran's leadership has been very wary of any gathering, whatever their 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.”

On Monday, 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 web site Tabnak.

State TV has aired statements by some arrested in previous water fight crackdowns, admitting they were motivated by “foreign invitations.” Some confessed they were 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 Iranian 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.

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.

Iran frequently accuses the United States and Iranian opposition groups in exile of fomenting opposition activity on its soil.

The protests sparked by Ahmadinejad's re-election, which opponents said was fraudulent, was the biggest challenge in 30 years to Iran's Islamic clerical rule. But security forces heavily crushed the wave of protests, and since then the opposition has been unable to return to the streets.

Cracking down on water-gun games reflects the leadership's wariness of any sign of opposition sentiment.

But even some conservatives who are strong supporters of Islamic rule thought arresting young people was going too far.

“I feel bad when I see some youth were detained for water fights. Those who support such detentions think the Islamic system is somehow very fragile,” said Mohammad Reza Zaeri, a conservative cleric, on a state TV talk show recently.

Lawmaker Mohammad Hossein Moghimi, another conservative, said young people were holding water fights because of a lack of other entertainment and because of so many other restrictions on them.

“Sometimes, we make it too hard for people and constrict them, so they react,” he said. “We have to make people comfortable.”