TEHRAN, Iran — Every night, millions of young Iranians are engaged in subversive, and in some cases, scandalous activity.

According to the country's Mobile Communications Co., more than 20 million text messages are sent within the country every day, with the largest number of messages sent between 10 p.m. and 1 a.m.

That's when the youth in the country communicate with one another, often transmitting messages that would meet with disapproval of conservative authorities.

Using cell phones to send text messages has reached epidemic proportions in the country, creating an invisible but powerful social network that is all but invisible to the authorities.

It's become the preferred way to conduct political and cultural discourse, filling the gap left by the dearth of free and independent media.

Political humor, much of it aimed at President Mahmoud Ahmadinejad, makes up a surprisingly large percentage of the conversations.

One joke making the rounds this summer during the country's gasoline shortage, read: "Ahmadinejad was asked what people without gasoline should ride on. He replied that they should ride on the 17 million who voted for him."

The message was soon on blogs all across the country, and was even cited by the Voice of America's Persian service.

Dr. Babak Khabiri, an expert on social affairs, says that text messaging is one of the few methods of expressing views and criticisms that has not been subject to restrictions.

"There is no need to conduct an opinion poll to keep a finger on the pulse of Iranian society," he said. "It's enough to collect and analyze day-to-day text messages."

Text messages have become an effective if unauthorized method of keeping up on the news, such as Ahmadinejad's appearance at Columbia University in September and reports that the government intended to launch a campaign to make young women adhere strictly to the Islamic dress code.

Text messages also have been responsible for the rapid spread of misinformation. One false report, claiming that gasoline rationing had been postponed for a day, led to mile-long lines of vehicles at service stations across the country. Another false report — that Farah Pahlavi, the widow of the late Shah of Iran, had died — even fooled some of the country's legitimate news agencies.

Earlier this year, there were reports of police checking the contents of young people's phones. There was even one media report that someone had been arrested and convicted of "keeping and sending immoral text messages."

Those reports initially appear to have led to a decline in text messaging. But the pace of texting appears to have resumed after widespread criticism in the media of such a policy, coupled with denials by police officials that such an arrest took place.

The government, at least for now, seems to have come to terms with the phenomenon, even if the flow of messages represents a direct challenge to the state and the political and social restrictions that are in place.

Texting in Iran today amounts to a popular struggle to talk freely about politics and break social taboos in a tightly controlled environment.

And for the moment, at least, it appears that nothing is safe from criticism or ridicule.

Parisa Dezfoulian is a journalist and political analyst in Iran who writes for The Institute for War & Peace Reporting.