As the rioting in England subsided, Prime Minister David Cameron announced he was looking for a way to clamp down on the social media he believes helped orchestrate it. “Everyone watching these horrific actions will be struck by how they were organized via social media,” Cameron told Parliament during an emergency session yesterday. “And when people are using social media for violence, we need to stop them.”

The statement was a call to arms for free-speech advocates. “It’s almost impossible to do fairly,” said Jim Killock, executive director of Open Rights Group, one of several organizations to condemn the statement. The ubiquity of communications technology makes banning someone from a single service futile, he says, and implementing a blanket ban would put innocent lives at risk by cutting off their communication during an emergency. He calls the idea “stupid,” but he sees it as an ominous sign of Cameron’s plans.

What’s more likely than censorship, says Killock, is that Cameron will seize the opportunity to extend his government’s surveillance powers. “It may be that they try to dust off the Labour Party’s Interception Modernization Program,” a plan to store every email, Web-page visit, and phone call made in the U.K. The plan was expensive and controversial, and the Liberal Democrats pledged to prevent it in their coalition agreement with the Conservatives. But last year the plan appeared again, buried in the Strategic Defense Review. “The administration hasn’t abandoned Labour Party's plans,” says Killock. “They just need to discuss it quietly behind the scenes.”

For the time being, British law enforcement does seem more interested in using social media as a surveillance tool than they are in censoring it. The Greater Manchester Police were confident enough in their ability to use social media to catch rioters that they warned them over Twitter: “If you have been using social networking sites to incite disorder, expect us to come knocking on your door very soon,” the department tweeted.

Over a dozen people have already been arrested for messages they’ve sent online. Two men from Lancashire and another from Sussex have been charged for using Facebook to encourage disorder, and an 18-year-old man was arrested after he tweeted a picture of himself posing with looted goods.

In Britain, it is a criminal offense to incite criminal activity through media, says John Spencer, a criminal-law professor at the University of Cambridge, though this is the first time he has heard of anyone being charged for incitement using digital media.

Suspects have been arrested for incitement via BlackBerry Messenger (BBM) as well. But unlike Twitter and Facebook, messages sent over BBM are not public. The free messaging service allows users to send encrypted texts to several contacts, making it a popular organizing tool for activists and, apparently, rioters. Police have blamed the messaging service for helping rioters coordinate their looting on a massive scale.

Research in Motion (RIM), the company that makes BlackBerry, has said they will assist authorities “in any way we can,” but did not elaborate and did not respond to further requests for comment. Under the U.K.’s Regulation of Investigatory Powers Act, that assistance could include giving law enforcement traffic data without a warrant—the time messages were sent, where they were sent from, and the name of the sender. If law enforcement asks for the content of the messages, RIM could demand a warrant—but they don’t have to.

Killock worries that police getting customer data directly from RIM, rather than going through the courts, sets a dangerous precedent. “If police and private companies develop a habit of handing over information about people who are rioting, that will set a precedent that could be carried over to political activities.” As far as police are concerned, there is not a major difference between the recent riots and the austerity protests of several months ago.

Several arrests indicate police may be getting their RIM data already. A 27-year-old man and an 18-year-old woman were charged with using BBM to incite violence. In Northampton an 18-year-old man has been charged for urging 140 people to bring “bats and weapons” to Abington Park over BBM. In that case police were shown the message by one of the recipients, but police have not said how they obtained the other messages.

Scotland Yard has been making use of photo sharing as well. They quickly set up a massive Flickr archive of riot photographs and asked the public to identify the suspects. They have plenty of footage to draw on, as the U.K. has one of the most extensive networks of closed-circuit television (CCTV) cameras in the world. A recent study counted 1.85 million cameras in the nation, and a 2007 study estimated London to have one camera for every 14 residents, making it “the most watched city in the world.”

Similar crowd-sourced lineups appeared after the Stanley Cup riots in Vancouver, but the Vancouver Police Department discouraged them out of fear they would promote “vigilante justice.” Instead, they asked people to send the department their riot pictures and videos, and received so many that their website crashed.

Scotland Yard may be trying to move beyond crowd sourcing. Chief Constable Andy Trotter of the British Transportation Police told the Associated Press that Scotland Yard has been testing their new facial-recognition software, intended for the Olympic Games. (A spokesperson for the Metropolitan Police confirmed they have facial-recognition technology but refused to comment on an ongoing investigation.)

Facial-recognition technology has improved drastically over the last several years. Last week at the Black Hat computer-security conference in Las Vegas, Carnegie Mellon researcher Alessandro Acquisti used an iPhone camera, the face-recognition program PittPatt (acquired by Google last month), and a database of Facebook profiles to match his students’ faces to their identities. It worked a third of the time. Human recognition is better, says Acquisti, but automatic identification is fast improving.

Even with their new facial-recognition software, U.K. police may be looking overseas for help. Sean Mullin, the president of Massachusetts-based BI2, which makes facial-recognition technologies, says he has been barraged with calls since the riots began.

They’re especially interested in the MORIS device, which attaches to smart phones and allows law enforcement to scan a suspect’s face, fingerprints, and iris. The device, which stands for Mobile Offender Recognition and Information System, matches data against existing criminal databases. Not that it couldn’t draw on public databases like Facebook, provided the pictures were of the right quality, says Mullin, but there are “privacy concerns.” The MORIS is scheduled to be released to 40 law-enforcement agencies in the U.S. this fall.

The step from crowd-sourced photo identification to automated facial recognition might seem like a small one, but it raises new privacy concerns, says Helen Nissenbaum, a professor of media at New York University. Crowd-sourcing identification requires great effort, so it is currently used only for serious crimes. As automation continues to improve, she says, it will be tempting to use it for a wider range of infractions, and we will have to decide how much anonymity we want to preserve in public spaces. “The technology isn’t going to draw the line for you.”

The day before Cameron mentioned blocking social media, he expressed disdain for such lines: “Picture by picture, these criminals are being identified and arrested, and we will not let any phony concerns about human rights get in the way.”