After the overthrow of Hosni Mubarak in Egypt and this summer's looting in England, there is no longer any doubt about the speed with which large crowds can be mobilized on to the streets. As flash-mobbing morphs into flash-robbing, the attention of British authorities is turning to the mobile phones and social media that empower everything from benign groups dancing in railway stations to the vandalism of entire high streets.

In the week of the England riots, the operator of San Francisco's subway pulled the plug on its own mobile network for three hours, even preventing passengers from making emergency calls. (photo: Steve Rhodes)

During the riots, two London MPs called for a BlackBerry Messenger curfew, proposing a 6pm to 6am shutdown of the service being used by gangs to organize looting. It was not implemented but in the aftermath, a review of police powers, including those to intervene in mobile communications, was announced. Theresa May is to meet Twitter, Facebook and BlackBerry maker Research in Motion (Rim) to discuss tighter controls, and the prime minister has warned: "When people are using social media for violence, we need to stop them."

Companies and politicians are being forced to rethink the extent to which governments or the police should be able interfere with communication networks. This year has seen at least two heavy-handed interventions. The Egyptian government ordered the closure of Vodafone's network, and those of other operators, during the Tahrir square uprising. Vodafone remained down for 24 hours, and when service resumed the company was strong-armed into sending out pro-government text messages urging demonstrators to stay at home.

In the week of the England riots, the operator of San Francisco's subway pulled the plug on its own mobile network for three hours, even preventing passengers from making emergency calls. In an echo of the killing that sparked the Tottenham riots, the San Francisco shutdown was designed to prevent a demonstration over the fatal shooting of an unarmed man by a transport policeman. Last year, Vodafone's partner company in Bahrain complied with restrictions forcing sim card users to register their personal details. More than 400,000 of those who did not were cut off.

While new companies such as Twitter are beginning to work out where they stand on such issues, the mobile phone networks are used to operating under heavy regulation and have well established systems for co-operating with police. O2, for example, has a high-security police liaison team in Slough which employs 30 staff who are constantly in touch with police over anything from missing persons to murders.

"Switching off any network element or any device should be a last resort," says Mike Short, European technology vice-president for Telefónica, which runs the O2 network. O2's view, and that of its fellow carriers, can be summed up by its advertising slogan: "We're better connected."

There is only one reported case of a UK network being closed by police. During the 7/7 London suicide bombings, O2 phone masts in a 1km square area around Aldgate tube station were disconnected for a number of hours.

Police have an emergency power to order masts to be put out of action known as MTPAS – Mobile Telecommunication Privileged Access Scheme. The move has to be approved by Gold Command, by the officers in highest authority during a major incident, and is designed to restrict all but emergency service phones with registered sim cards from making calls. But a shutdown can have dangerous knock-on effects. Short says that phones within the Aldgate zone automatically sought a signal from live masts outside it, overloading them and causing a network failure that rippled out "like a whirlpool".

On the day, other networks were simply overloaded as Londoners sought reassurance and information. Vodafone alone experienced a 250% increase in call volumes. MTPAS was introduced so emergency services could still contact each other during outages but could in theory be used to stop troublemakers organizing.

"I would be extremely slow to suggest that networks or applications allow communications to be shut down because you create as many problems as you solve," says Neil Brown, telecoms partner at law firm Eversheds. "You need to control the users rather than the whole network, because the whole network can work for the good."

LikeAs with many Londoners, during the riots Brown spent a number of hours on the phone to his son, who lives in Barking, calming him as hooded youths paraded in the street below.

Police have already told MPs that they contemplated seeking authority to close down Twitter and other services, but that monitoring communications on these channels allowed them to identify the next targets and send officers to protect the Olympic site, Westfield shopping center and Oxford Street.

More refined network closures are possible. At least two UK operators, including O2, have the ability to prevent just data communication – use of the internet from a phone. This allows voice calls and text messaging from within the zone, but shut down cannot always be done mast by mast.

Each internet gateway serves thousands of masts. Shutting one or two gateways could black out an area as big as south London. If police wanted to block mobile internet across an entire city center, the outage would most likely affect neighboring cities too.

Intriguingly, Short says it would be possible for his network to bar just BlackBerry phones. But this could not be done in a targeted way: mast by mast or even city by city. The exclusion would have to be across a major part of the country. Mobile operators can, under warrant, provide useful information to track down offenders after an event. Anyone requesting information must be accredited, and requests are logged, monitored and reported annually in a general, statistical way by the chief surveillance commissioner, currently the former judge Sir Christopher Rose.

While they do not have the technology to reveal the content of calls or texts, network operators can track web pages visited, geographical movements at particular times, find the registered user's name and address, and the call log. Networks only have the resources to track a small number of phones geographically in real time, so that this kind of evidence is usually retrospective and cannot prevent a riot. It was, however, used to solve the Soham murders case and is helping in looter prosecutions.

Perhaps the most interesting real-time information is on mass movement patterns. Networks have installed systems to detect crowds, in order to manage capacity. Phones signal where they are every 15 minutes, which means the gathering of a big crowd, or its direction and speed, can be rapidly pinpointed.

O2 is talking to the police about using this technology, known as cell congestion monitoring, during the Olympics. Police could be mobilized quickly to prevent crushes or stampedes.

"We don't want to be big brother about this," says Short. "The service is primarily for our customers, if we can help the police under law that is also fine but we don't want to change our systems just for a few incidents, however great they are."

He says the industry would welcome guidance in the next Communications Act, for which a green paper is expected by Christmas. In the UK as in most other nations, the government has the right to pull the plug if it sees fit, but mobile phone operators are coming under pressure to promote best practice.

Access, which lobbies for communication freedom, suggests a set of principles that would include only closing networks as a last resort for short periods, ensuring all phones can call emergency services during a shutdown, and refusing to act as a spokesperson for a regime.

The group's director Brett Solomon warns: "These channels of communication have been highly useful to British citizens in organizing clean-ups and getting information about the safety of their families and friends."

And as he points out, any draconian new measures in the UK would undermine the fight for open communication that western democracies championed during the upheavals in the Middle East.