"His health condition deteriorated after the hunger strike and he is now hospitalised," said 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 Wednesday morning, it said, adding that he was still refusing to eat.
His son Mohammad also confirmed the news on Twitter.
"At 1:00 AM, Thursday father was sent to hospital due to the hunger strike. Pray a lot," he said.
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"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.
He and fellow reformist leader Mir Hossein Mousavi were candidates in Iran's disputed 2009 presidential election which sparked months of mass protests over claims they were rigged in favour of hardline incumbent Mahmoud Ahmadinejad.
In March, Iran sentenced Karroubi's eldest son, Hossein Karroubi, 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, a reputed moderate, calling for a trial.
The 79-year-old Karroubi was hospitalised earlier this month and underwent a heart operation, Sahamnews reported.
A reformist MP, Mostafa Kavakebian, raised Karroubi's hunger strike in parliament yesterday, calling for action from the health and intelligence ministries.
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