After four nights of lawlessness that has upended British society and seen 1,200 alleged looters and arsonists swept off the streets, the government is also targeting a digital culprit: social media.

Governments from China to the authoritarian regimes challenged by the Arab Spring have sought to control social networking sites, fearing their power to connect and organize dissidents hungry for democracy. But Britain is weighing an unprecedented move to intervene in the personal communication of its citizens after concluding just the opposite: that social media, including BlackBerry Messenger and Twitter, are undermining its vibrant democracy.

The plan touched off an immediate firestorm in Britain’s thriving social media community, igniting charges of an assault on freedom of speech. Prime Minister David Cameron, however, made clear that he felt the greater threat was allowing violent speech to circulate.

“Everyone watching these horrific actions will be struck by how they were organized via social media,” Cameron said in an emergency session of Parliament on Thursday, during which he announced that officials were working with the intelligence services and police to look at how and whether to “stop people communicating via these Web sites and services when we know they are plotting violence, disorder and criminality.”

Cameron said: “Free flow of information can be used for good. But it can also be used for ill. And when people are using social media for violence, we need to stop them.”

Police in Britain have been monitoring social networking sites — and pouncing. Authorities in England and Scotland, for instance, have arrested more than a dozen youths on suspicion of using the Internet and text messages to incite unrest. In Greater Manchester, hit hard by rioting Tuesday night, the police fought fire with fire, issuing this warning from an official police Twitter account: “If you have been using social networking sites to incite disorder, expect us to come knocking on your door very soon.”

The Twitter universe in Britain was among the first to respond to Cameron’s announcement, with irate comments flooding the nation’s digital space. Spoofing Sky News — Fox News’s sister network in Britain — one Twitter user employed classic British humor as political statement, sending out a fake bulletin: “Breaking: Sky News understands David Cameron has been in talks with the Chinese government to share web-filter technologies.”

Furthering a nascent debate here over civil rights in the aftermath of the riots were additional emergency measures outlined by Cameron.

The government announced that it would start slapping “gang injunctions” — now used for adults — on underage teens, using court-ordered restraining orders to ban them, for instance, from wearing gang colors or walking around their neighborhoods with attack dogs.

Police have also been authorized to force suspicious-looking people who have their faces covered by, say, bandannas and “hoodies” — the uniform of British hooligans and gang members — to identify themselves. The government is also reviewing the possibility of imposing curfews.

With police forces here stretched to the limit, Cameron said the government will draft contingency plans to have the army take over backroom tasks for Scotland Yard and other forces to free them up for street patrols in the event that it becomes necessary. Although he said such steps were not needed at the moment — city streets were largely quiet across Britain on Wednesday and Thursday, possibly in part because of the beefed-up police presence — some in the opposition criticized the suggestion.

“Whether it’s a popular thing to say or not, a further militarization of the situation will not help and will bring things to an even worse level,” warned Diane Abbott, an opposition Labor Party lawmaker from the hard-hit London neighborhood of Hackney.

But underscoring how deeply the riots had rocked this nation, lawmakers across the political spectrum were condemning the role of social media in the riots, calling for a way to blunt their use as tools of violence.

Rampaging youths, authorities say, used BlackBerry Messenger, Twitter, Facebook and other social media to organize disturbances, sharing meeting times and locations and, in some cases, openly calling for the spread of criminality. BlackBerry Messenger, which does not charge for text messages, was particularly popular among the largely poor youths who rioted.

One BlackBerry text circulating in London on Monday night urged rioters to pour into the Enfield neighborhood of north London, where a Sony distribution center was set ablaze. Another text called on rioters to attack stores in the famous shopping district of Oxford Circus: “Everyone run wild, all of London and others are invited! Pure terror and havoc & Free stuff. Just smash shop windows and cart out da stuff u want!”

But there were also comments, including several on Facebook, that rather than organizing riots, simply spoke out in their support. It’s unclear whether those would be seen as inciting violence and thus be targets of a British government campaign.

“The riots are 4 us to show the police that they cnt bully us,” said one posting on the Facebook page of a user named Priceless King.

Experts warn, however, of technical issues in blocking or interpreting encrypted BlackBerry instant messages, which operate on a special network. Theresa May, Britain’s home secretary and the minister in charge of policing, said she would meet with representatives from social media companies to come up with a plan, though the government appeared to be testing the waters of public opinion before pressing ahead.

May said, however, that social media have clearly been used “to coordinate criminality and stay one step ahead of the police.”

BlackBerry officials would not directly comment on the government’s announcement. But the company said in a statement: “As in all markets around the world where BlackBerry is available, we cooperate with local telecommunications operators, law enforcement and regulatory officials.”

But student activists — including those who in December helped organize a political protest against budget cuts that turned violent after a breakaway group vandalized central London businesses — decried the move as a full-on attack on freedom of expression. They questioned whether such measures would be used against them as well, warning that the government could be going down a dangerous road toward silencing voices of social dissent.

“This puts Cameron in very poor company internationally,” said Martin Young, 22, who took part in the December student protests. “This is the same reaction by the Middle East and North African governments, which we condemned for having no respect for privacy and freedom of communications.”

Special correspondent Rebecca Omonira-Oyekanmi contributed to this report.