Role of Facebook, Twitter and BlackBerrys in UK violence to be examined by home secretary Theresa May

Facebook has responded to David Cameron's calls for a clampdown on social networking sites by saying it has already actively removed several "credible threats of violence" related to the riots across England.

The prime minister told parliament on Thursday that Facebook, Twitter and Research in Motion (Rim), the maker of BlackBerry devices, should take more responsibility for content posted on their networks, warning the government would look to ban people from major social networks if they were suspected of inciting violence online.

The home secretary, Theresa May, is to hold meetings with the three companies within weeks.

The police have promised to track down those suspected of inciting the violence on Twitter, but much of the planning for the disturbances took place in the relatively private world of the BlackBerry Messenger service.

A Facebook spokeswoman said: "We look forward to meeting with the home secretary to explain the measures we have been taking to ensure that Facebook is a safe and positive platform for people in the UK at this challenging time.

"In recent days, we have ensured any credible threats of violence are removed from Facebook and we have been pleased to see the very positive uses millions of people have been making of our service to let friends and family know they are safe and to strengthen their communities."

Mike Conradi, partner and telecoms specialist at the London law firm DLA Piper, said that emergency measures to stop rioters communicating on social media sites would require legislation and threaten free speech.

Conradi said: "What David Cameron appears to be wanting is a police power to trawl through millions of messages – ideally in real time – to prevent possible criminal activity. I don't believe that any such power exists and nor would I want there to be one. Parliament would have to pass new legislation and I would certainly warn against that. That gets the balance wrong in terms of free speech and security.It would certainly put the UK in a difficult position in terms of talking to authoritarian regimes and trying to convince them not to turn off their networks."

Current powers allow Rim and others to identify people who may be worth further investigation and potential prosecution without looking at the contents of their messages.

Cameron's move to curb social media was backed by the opposition. Ivan Lewis, the shadow culture secretary, said: "Free speech is central to our democracy but so is public safety and security. We support the government's decision to undertake a review of whether measures are necessary to prevent the abuse of social media by those who organise and participate in criminal activities."

The only organisation which regularly removes illegal content from websites within hours of its discovery is the Internet Watch Foundation, which combats images of child abuse. Funded by internet service providers, mobile operators and other web businesses, it has no legal powers. Any co-operation is voluntary.

It uses "notice and take down procedures", which have been widely adopted in the US and Europe to protect internet publishers from being held liable for hosting illegally copied material. Most websites, if contacted with a complaint about their content, take down the material.

Efforts to control messaging during riots are likely to focus on social media rather than mobile phone companies, because there is no simple procedure for police to cut off individual phones at short notice.

Vodafone Group communications director Matt Peacock said: "It is not possible to cut off access to an individual subscriber if the police don't even know who that person is, as would seem highly likely in the middle of a riot with hundreds of masked youths running around the streets.

"In any case, the police must follow a legal process in order to require operators to disclose individual subscriber information. It's an important process, designed to protect customers' privacy, and it's proven and robust. However, it isn't designed to operate in the context of this kind of fast-moving and highly volatile civil disorder."

Powers do exist in the UK and most other countries to order the shut down of entire networks or individual base stations, blocking all traffic in a particular area. These are seen as a last resort.

Vodafone and other carriers were widely criticised for shutting their Egyptian networks during anti-government demonstrations.