Leading Iranian opposition figure Mirhossein Mousavi, under house arrest for more than a year, was taken to hospital on Thursday for treatment for a heart problem, one of his former senior advisors said, Reuters reported.
Mousavi and fellow reformist Mehdi Karoubi ran for election against President Mahmoud Ahmadinejad in June 2009 and became figureheads for the large protests that followed by Iranians who accused authorities of rigging the vote to bring back the hardline incumbent.
The government denied any vote wrong-doing and said Iran's foreign enemies had plotted to overthrow the country's leaders by stirring up the protests, the biggest opposition demonstrations since the 1979 revolution.
"Mousavi was taken to the hospital this morning after his blood vessels became blocked," said Ardeshir Amir Arjomand, a senior advisor to Mousavi during his presidential campaign and leading exiled opposition figure.
"He has not been feeling well since last night, but the security forces did not take him to the hospital until this morning because they wanted to install cameras there," Arjomand told Reuters by telephone from Paris.
Mousavi, his wife Zahra Rahnavard and Karoubi have been held incommunicado since February last year when the two leaders called their supporters onto the streets for a rally in support of uprisings in the Arab world - the first demonstrations by their pro-reform "Green movement" since street protests were crushed by security forces at the end of 2009.
Since then, the two opposition leaders have not been seen in public.
Months after the 2009 election, members of parliament called for the pair to be tried and hanged but the authorities chose to isolate rather than officially arrest them, wary of angering their supporters.
Arjomand said the families of the two opposition figures had been under "severe and inhumane pressure" in recent months.
"Mr. Mousavi did not suffer from any illness prior to his house arrest. He and his family have been under a lot of pressure but he has stood firm ... The same is true for Mr. Karoubi and his family."
Arjomand also called on United Nations Secretary General Ban Ki-moon and other leaders to demand a meeting with the imprisoned opposition leaders when they travel to Iran in the coming week for a summit of 120 developing nations.