Iran has stepped up pressure on the opposition, arresting a dissident cleric and warning against using SMS and email messages to organise anti-government protests, reports said on Friday.

Mid-ranking cleric Mohammad Taghi Khalaji, who was close to late dissident Grand Ayatollah Hossein Ali Montazeri, was arrested at his home in the city of Qom on Tuesday, reformist website Norooznews said.

“The (reason for) his arrest is not known yet but he had on several occasions backed protests against election results” which saw President Mahmoud Ahmadinejad returned for a second term amid allegations of massive fraud, the report said.

The detained cleric’s son, Mehdi Khalaji, is a senior research fellow at the Washington Institute for Near East Policy and two members of the US House of Representatives condemned the arrest.

“The regime’s intimidation and imprisonment of clerics critical of the current government must cease at once,” Ileana Ros-Lehtinen and Howard Berman said in a joint statement.

Iranian clerics backing the opposition have faced mounting pressure by hardliners since the death of Montazeri, whose December funeral was attended by thousands of opposition supporters.

An outspoken critic of the Islamic regime he helped create, Montazeri joined the opposition by calling for protests and branding Ahmadinejad’s government as illegitimate.