Roxana Saberi

 Reformist websites reported that Mr Mousavi, 70, who has been under house arrest for 555 days, was admitted to a specialist cardiac unit in Tehran under intense security on Thursday morning for treatment to a blocked artery.

He underwent a three-hour angiography, a radiography procedure designed to take images of the blood vessels, and was later transferred to a coronary care unit.Mr Mousavi, the losing candidate to President Mahmoud Ahmadinejad in a 2009 election widely denounced as fraudulent, was kept in hospital overnight, accompanied by his wife, Zahra Rahnavard, who is also under house arrest.

Tens of Revolutionary Guards and plain-clothed intelligence agents are standing guard over the two, the website Kalemeh reported. CCTV cameras are said to have been installed throughout the unidentified hospital in advance of the reformist leader’s admission. He is understood to have been barred from receiving visitors.

News of Mr Mousavi’s health problems are an unwelcome distraction toIran’s Islamic regime in the run-up to the opening of the Non-Aligned Movement conference, scheduled to open in Tehran on Sunday.

Iran’s leaders are keen to use the event – which is expected to be attended by leaders and representatives of 120 countries – to showcase the nation’s credentials as a leader of the developing world in opposition to the West. Ban Ki Moon, the UN secretary-general, has announced he will attend the conference.

The Green Voice of Freedom website, which supports Mr Mousavi, speculated that his admission to hospital was linked with a request it said Mr Ban had made to meet him.In a sign of the raised stakes, Mr Mousavi’s Paris-based spokesman, Ardashir Amir-Arjmand, said Iran’s supreme leader, Ayatollah Ali Khamenei, bore ultimate responsibility for his well-being.

Little had previously been heard of Mr Mousavi’s state of health since he and Mrs Rahnavard, were put under house arrest in February 2011 after urging Iranians to stage mass protests similar to those that had triggered the Arab Spring.

Mr Mousavi’s fellow reformist, Mehdi Karroubi, was also placed under house arrest and remains in detention.

Until then, Mr Mousavi, a former prime minister and pillar of the 1979 Islamic Revolution’s establishment, had been nominally free but with his movements closely monitored and severely curtailed.

He and Mr Karroubi were frozen out of Iran’s leadership circle after backing mass protests against Mr Ahmadinejad’s 2009 election victory.The protests were crushed in a fierce clampdown that resulted in the deaths of scores of demonstrators and the arrests of thousands of opposition activists. Ayatollah Khamenei accused Mr Mousavi and Mr Karroubi of spearheading a “sedition” aimed at toppling the Islamic system.