Iranian opposition leader Mehdi Karroubi, under house arrest for the past six years, was hospitalised Thursday after going on hunger strike to demand a trial, his family told local media.

"His health condition deteriorated after the hunger strike and he is now hospitalised," said the Sahamnews website, which is affiliated to Karroubi's family.

He was admitted to Shahid Rajaei hospital due to high blood pressure, less than 24 hours after beginning a hunger strike, it said, adding that he was still refusing to eat.

Karroubi, 79, and fellow reformist leader Mir Hossein Mousavi were candidates in Iran's disputed 2009 presidential election which sparked months of mass protests over claims that the polls were rigged in favour of hardline incumbent Mahmoud Ahmadinejad.

Both were placed under house arrest in 2011 for their role in the protests -- which were brutally suppressed by the regime -- but have never been charged.

Karroubi's wife Fatemeh told Sahamnews that his first demand was the removal of intelligence ministry agents and security cameras that had been recently installed inside their home, which she said "has no precedent before or after the (1979 Islamic) revolution in any house arrest".

"Second... in case of continuation of the house arrest, they should arrange a public trial," she said.

Karroubi "does not expect a fair trial" but wants it to be public and would respect the verdict, she added. 

In March, Iran sentenced Karroubi's eldest son Hossein to six months in prison for "propaganda against the regime" after he published a letter that his father had written to Iran's current president, Hassan Rouhani, considered a political moderate, calling for a trial.

His other son Mohammad has provided updates on Twitter, which has been banned in Iran since the 2009 protests but is widely accessed using privacy software.

"At 1:00 AM Thursday, father was sent to hospital due to the hunger strike. Pray a lot," he wrote.

- Rouhani support -

Karroubi's failing health -- he underwent a heart operation earlier this month -- poses a potential problem for the Iranian regime, with fears that it could provide a lightning rod for renewed protests. 

Already, lawmakers have used the hunger strike to renew their criticism of the house arrest.

"The logic of some of the security and intelligence officials, which says that you must repent in order to be released, has neither a rational nor a religious basis," said outspoken MP Ali Motahari on his Instagram page.

Rouhani won a resounding re-election victory in May, in part by rallying reformists and vowing to win the release of Mousavi and Karroubi.

In the relatively free atmosphere that attends the short campaigning period before Iranian elections, reformist crowds openly chanted their names at Rouhani rallies.

But hardline judiciary chief Ayatollah Sadegh Larijani threw cold water over Rouhani's promises shortly after the election.

"Who are you to end the house arrest?" Larijani said in May.

There has been far less political unrest in Iran in recent years -- in part due to the crackdown on the opposition, but also thanks to a relative easing of social pressure under Rouhani.