Mehdi Karroubi, right, and Mir Hossein Mousavi have used the internet to continue their criticism of the violent crackdown that followed June's election. Photograph: AP


Team set up with view to pressing charges against those 'spreading lies' is seen as attempt to gag Mousavi and Karroubi


Iran has moved to block the last remaining outlet of expression for the country's political opposition with the launch of a special force to police the internet.

A 12-member team reporting to the chief prosecutor will scour websites with a view to pressing charges against those judged to be "spreading lies" and "insults" against the Islamic system. Members will include police and personnel from other, unspecified, parts of Iran's security apparatus.

The move is seen as partly targeting Mir Hossein Mousavi and Mehdi Karroubi, the two defeated reformist candidates in June's disputed presidential election, as well as political activists.

Colonel Mehrdad Omidi, the head of the web crime unit, said cyber-crimes would be treated as seriously as armed robbery. He pledged to intervene in "political matters on the internet, should there be an illegal act".

"Given the spread of internet use, police must confront crimes taking place in the web atmosphere," the Iranian media reported him as saying. "A special committee has been set up to monitor the internet and deal with crimes ... such as fraud ... insults and the spreading of lies."

While vowing to tackle internet crimes in general, the unit's political nature is indicated by the emphasis on "insults and lies" – a term often used to describe opposition statements since the election.

Mousavi and Karroubi have used newly established websites to continue their criticism of the violent crackdown that followed Mahmoud Ahmadinejad's victory, which they say was obtained through fraud. The Mousavi-linked Kalemeh and the Karroubi-affiliated Tagheer have continued voicing their views even after newspapers supporting them were closed. Pro-regime hardliners have repeatedly demanded the arrest of the pair for statements published online in their names.

They have also begun posting filmed interviews online to overcome a lack of access to state-controlled television.

In a statement posted on yesterday Mousavi called for the release of all political prisoners. "We want guarantees for healthy elections. We want freedom of the press and speech. ... Islam is the religion of logic, not the religion of breaking pens and mass closures of newspapers," he said.

Access to the internet is already restricted in Iran. An estimated 10m websites deemed socially or politically offensive have been blocked – prompting the Paris-based media watchdog Reporters Without Borders to list Iran as one of the world's 13 worst internet "black holes".

A registration system has also been introduced for bloggers, many of whom have been arrested and jailed. Three years ago, the government banned high-speed broadband internet for home users in an attempt to prevent the downloading of western films and pornography.

But the new crime unit represents a significant escalation of a clampdown on cyberspace that has intensified since the mass streets protests that greeted Ahmadinejad's re-election.

In the days after the June poll, internet speeds were reduced to a crawl while Facebook and Yahoo Messenger – used by many activists to communicate – were blocked. At the same time, it was disclosed that Iran had purchased equipment jointly produced by the German electronics firm Siemens and Nokia, the Finnish mobile phone manufacturer, to closely monitor internet and email communications. The revelations prompted a consumer boycott of the companies by pro-opposition demonstrators.