Shortly after Neda Agha-Soltan bled her life out on the Tehran pavement, the man whose 40-second video of her death has ricocheted around the world made a somber calculation in what has become the cat-and-mouse game of evading Iran’s censors. He knew that the government had been blocking Web sites like YouTube and Facebook. Trying to send the video there could have exposed him and his family.

Copies of the video, as well as a shorter one shot by another witness, spread almost instantly to YouTube and were televised within hours by CNN. Despite a prolonged effort by Iran’s government to keep a media lid on the violent events unfolding on the streets, Ms. Agha-Soltan was transformed on the Web from a nameless victim into an icon of the Iranian protest movement.

At one time, authoritarian regimes could draw a shroud around the events in their countries by simply snipping the long-distance phone lines and restricting a few foreigners. But this is the new arena of censorship in the 21st century, a world where cellphone cameras, Twitter accounts and all the trappings of the World Wide Web have changed the ancient calculus of how much power governments actually have to sequester their nations from the eyes of the world and make it difficult for their own people to gather, dissent and rebel.

Iran’s sometimes faltering attempts to come to grips with this new reality are providing a laboratory for what can and cannot be done in this new media age — and providing lessons to other governments, watching with calculated interest from afar, about what they may be able to get away with should their own citizens take to the streets.

One early lesson is that it is easier for Iranian authorities to limit images and information within their own country than it is to stop them from spreading rapidly to the outside world. While Iran has severely restricted Internet access, a loose worldwide network of sympathizers has risen up to help keep activists and spontaneous filmmakers connected.

The pervasiveness of the Web makes censorship “a much more complicated job,” said John Palfrey, a co-director of Harvard’s Berkman Center for Internet and Society.

The Berkman Center estimates that about three dozen governments — as widely disparate as China, Cuba and Uzbekistan — extensively control their citizens’ access to the Internet. Of those, Iran is one of the most aggressive. Mr. Palfrey said the trend during this decade has been toward more, not less, censorship. “It’s almost impossible for the censor to win in an Internet world, but they’re putting up a good fight,” he said.

Since the advent of the digital age, governments and rebels have dueled over attempts to censor communications. Text messaging was used to rally supporters in a popular political uprising in Ukraine in 2004 and to threaten activists in Belarus in 2006. When Myanmar sought to silence demonstrators in 2007, it switched off the country’s Internet network for six weeks. Earlier this month, China blocked sites like YouTube to coincide with the 20th anniversary of the Tiananmen Square crackdown.

In Iran, the censorship has been more sophisticated, amounting to an extraordinary cyberduel. It feels at times as if communications within the country are being strained through a sieve, as the government slows down Web access and uses the latest spying technology to pinpoint opponents. But at least in limited ways, users are still able to send Twitter messages, or tweets, and transmit video to one another and to a world of online spectators.

Because of the determination of those users, hundreds of amateur videos from Tehran and other cities have been uploaded to YouTube in recent days, providing television networks with hours of raw — but unverified — video from the protests.

The Internet has “certainly broken 30 years of state control over what is seen and is unseen, what is visible versus invisible,” said Navtej Dhillon, an analyst with the Brookings Institution.

But taking pictures is an increasingly dangerous act in Iran. The police in Tehran confronted citizens who were trying to film near a memorial to Ms. Agha-Soltan on Monday.

Threatening people who have cameras is only the latest in a series of steps by the authorities. On June 12, the day a disputed presidential election set off the protests, the government summarily shut down all text messaging in the country — the prime tool that government opponents had been using to keep in touch — making new tools like Twitter and old techniques like word of mouth more important for organizing.

In the days that followed, Iran has tightened the spigot without closing it entirely. Even before the election, the country was known to operate one of the world’s most sophisticated Web filtering systems, with widespread blockades on specific Web sites. According to a spate of news reports in April, including one in The Washington Times, some of the monitoring technology was provided by Nokia Siemens Networks, a joint venture of Nokia, the Finnish cellphone maker, and Siemens, the German technology giant.

The day after the election, Iran’s state-controlled telecommunications provider completely dropped off the Internet for more than an hour, according to Renesys, an Internet monitoring company. Access was partly restored two days later, a Monday. YouTube said traffic to the site from within Iran was down about 90 percent last week, indicating that most — but not all — connections had been stopped or slowed. Facebook said traffic from Iran was down by more than half since the election.

Whether for political, social or financial reasons, Iran has been hesitant to shut off its sterilized Internet access entirely. Some have reasoned that a complete halt would hurt businesses.

Still, the off-and-on Web connections and government threats imposed a kind of self-censorship on some of the population, one that is also evident in other countries with authoritarian governments, Mr. Palfrey said.

Some Iranians have harnessed ways to bypass the system, relying in part on supporters around the world who are offering their computers as so-called proxy servers, which are digital safe houses that can strip out identifying information and allow Iranians to view blocked Web sites. Tor, a volunteer-run tool for masking Internet traffic that bounces Internet connections off three separate computers, said the traffic emanating from Iran over the course of the week increased tenfold.

Despite the crackdown, the videos and tweets indicate to many that broadly distributed Internet tools — and the spirit of young, tech-savvy people — cannot be completely repressed by an authoritarian government.

“You can’t take the entire Internet and try to lock it in a little box in your country, as China continuously attempts to do,” said Richard Stiennon, founder of IT-Harvest, a Web security research firm. “There are just too many ways now to find paths around blockages. They would have to ban the Internet entirely, or build their own network.”

That may not be so far-fetched. Experts say China is in its own league for filtering. Ethan Zuckerman, a colleague of Mr. Palfrey’s at the Berkman Center, said China has “baked in the censorship” for its citizens by building its own Web sites and tools. Recently it said it would require so-called Green Dam filtering software to be installed on all computers sold in the country, prompting a complaint from the United States government and most likely kicking off yet another round of cat-and-mouse.