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updated 7/24/2009 7:04:28 PM ET 2009-07-24T23:04:28

The tweets still fly and the videos hit YouTube whenever protesters take to the streets in Iran — even as the Internet battle there turns more grueling.

Authorities appear to be intensifying their campaign to block Web sites and chase down the opposition online, and the activists search for new ways to elude them.

Sites such as Facebook, Twitter and YouTube remain blocked, as they have been since Iran's political turmoil began following the disputed June 12 presidential election. Internet experts believe the government is going further — including tracking down computers from which images and videos of Iran's protests are sent out to the rest of the world. Activists fear their every move online is watched.

"We are really worried about this. To protect myself, I just limit my posts on social networks, my tweets and also I deleted some parts of my personal blogs and my other notes on the Web," one Iranian who regularly sends tweets about the election turmoil said in an e-mail to The Associated Press.

Another said, "Every site where people can gather and stay connected and share news and pics ... is blocked." Both agreed to e-mail interviews on condition of anonymity, fearing government retaliation.

Ways to bypass blocks
The government is believed to have been aggressively developing software and technology in recent years to strengthen its filtering and monitoring of Web sites. Since the election, a number of Internet experts are countering by providing Iranians with improved proxy systems and other programs to get around government blocks and escape detection.

"I think the Iranian government is learning quickly how to control and contain these things," said Andrew Lewman, executive director of The Tor Project Inc., based in Boston.

His group's free downloadable Tor program allows Internet users to work through a network of relays run by volunteers around the world to access blocked sites and hide what they are doing on the Internet. Active sessions using Tor in Iran have jumped from a few hundred before the election to thousands after, the nonprofit group said.

The Internet has been a key tool for Iran's opposition on two fronts. One is internal — to organize protests and exchange information. The other is external — to let the world know what is going on amid severe government restrictions that bar foreign media from reporting and taking pictures and video on the streets. The government has been actively trying to block online activists on both fronts.

Hundreds of thousands of Iranians held protests denouncing the election as fraudulent until security forces launched a heavy crackdown, arresting hundreds and killing at least 20 protesters. Throughout, activists took to the online messaging site Twitter to relay 140-character posts about what they were seeing and hearing. They furtively recorded video of police and members of the feared Basij militia riding on motorcycles through throngs of protesters, or photos of demonstrators bleeding from battles with government authorities.

One video in particular gripped Iran and the world: Images of 27-year-old Neda Agha Soltan bleeding to death after being shot on a Tehran street were viewed millions of times on YouTube, and her death became a rallying cry for opponents of the regime.

Internet still important to protests
Even after the crackdown crushed the large protests, the Internet has remained key. In two smaller protests organized in recent weeks, constant tweets reported on where the demonstrators were gathering. Despite the restrictions, videos quickly emerged on YouTube showing thousands of protesters clashing with police and Basij. At the same time, Internet news sites have become vital for tracking arrests of opposition politicians and activists, who are often picked up from their homes far from scenes of protests.

During the height of the protests, authorities cut off cell phone service and text messaging — which are all run by state-run firms and through government-owned towers — to break up communications for organizing rallies. Phone service has returned, though it cuts off in parts of Tehran when authorities believe a protest will be held. Texting has been slower to come back and is sporadic at best.

The government has also tried a more traditional route to subdue dissent: arrests. At least 34 journalists and bloggers were detained after the election, joining seven others already in prison, according to the Committee to Protect Journalists.

Around a quarter of Iran's 65 million people are believed to have Internet access. Iran has long used filtering to restrict certain news and political or pornographic Web sites. But since the election, the number of blocked sites has increased.

Besides Twitter and YouTube, the BBC's Farsi-language news site is still blocked, and Web sites associated with opposition leader Mir Hossein Mousavi — who says he won the election — are constantly shut down. In the last week, two new Mousavi sites have been created after others were targeted.

The day after the election, Internet traffic in and out of Iran came to a near total stop, according to research done by Arbor Networks, a Chelmsford, Mass.-based Internet security company.

The cause is not known, but the group says one explanation could be that the state-run Internet company all but shut down the network so all traffic could be run through filtering programs, which can only handle limited volume. In the week after the election — the latest figures available — traffic picked up again to about 70 percent of normal. In Iran, the Internet remains slow because of the brakes on traffic.

Given the secrecy with which the Iranian government operates, it's difficult to assess exactly what it is doing to monitor the Internet. A number of groups have sprung up to offer Iranians their online expertise, including on called NedaNet, in honor of the woman who died.

Morgan Sennhauser, a project coordinator for NedaNet, published a 31-page paper detailing the strategies and tactics the government is believed to be using.

The methods include blocking data from going to or from certain Internet Protocol addresses — the numeric identifiers for every computer connected to the Web. Another technique used, called packet fingerprinting, allows the government to judge certain characteristics of a packet of data to decide whether to block it or not, so that, for example, a firm's international transactions can go through but pictures of a protest cannot.

NedaNet, which describes itself as a group of independent "computer hackers and computer users," aims to set up proxy servers and other technology to enable Iranian users to make themselves anonymous and escape detection. Another proxy system that Iranians often use is Freegate, which was first developed by Chinese dissidents to get around Beijing's heavy Internet censorship.

"I do think that we're going to continue to counteract just about everything they can come up with, it's just a matter of time," Sennhauser said in e-mailed comments to the AP.

Gaurav Mishra, CEO of the social media research and strategy company 20:20 WebTech, said Twitter and Facebook do help get news out of Iran, but he warned against exaggerating their power to enact change.

"At best, these tools are catalysts, which are very important roles, but should not be overrated," he said. "To expect Twitter and Facebook on their own to make a fundamental change in that situation is expecting too much."

There is one method of communication that the government has been unable to stop. Every evening since the election, Iranians climb to their rooftops and scream "Allahu akbar," — "God is great" — a protest tactic used during the 1979 Iranian Revolution.

Copyright 2009 The Associated Press. All rights reserved. This material may not be published, broadcast, rewritten or redistributed.

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