The Iranian regime has arrested an elderly and ailing reformist while he underwent medical tests in a Tehran hospital in its latest attempt to repress protests against electoral fraud.

Unable to find him when they called at his home, officials tracked down Ebrahim Yazdi, the 78-year-old leader of the banned but officially tolerated Freedom Movement, as he was undergoing stomach tests and took him away to Evin prison, his family and colleagues say.

After helping in the 1970s to found the Freedom Movement, a group of intellectuals critical of the Shah's regime, Mr Yazdi was the foreign minister in Iran's first government after the 1979 revolution but has been sidelined since hardliners took control.

Mehdi Noorbaksh, Mr Yazdi’s son-in-law who lives in Harrisburg, Pennsylvania, confirmed that he was arrested at Pars Hospital at around 3pm and taken to Evin Prison, near the Iranian capital. Information was scant because phone lines had been cut off, he added.

Hadi Ghaemi, director of the New York-based International Campaign for Human Rights, said that Mr Yazdi was arrested in the intensive care unit.

So far 200 prominent journalists, opposition politicians and reformists have been rounded up by the regime, Mr Ghaemi said, including Mohammad-Reza Jalaipour, an Oxford student and noted Iranian analyst, who was arrested at the airport while trying to leave Iran with his wife, Fatemeh Shams. A plainclothes officer did not give a reason for the arrest, Shams told the BBC's Farsi website.

Hamid-Reza Jalaipour, Mr Jalaipour's father, vented his anger in an interview on the website, demanding: “Is it a crime to support Mousavi? That’s my only question now. Man, they have fallen to attacking people’s wives and children.”

Today hundreds of thousands of supporters of defeated presidential candidate Mir Hossein Mousavi were preparing for their sixth day of street demonstrations, laying aside their green campaign colours and wearing black to show mourning for the protestors already killed by the authorities. Officially the death toll is eight, but unofficial reports which cannot be confirmed suggest it may be higher.

The demonstrators’ willingness to defy bans, guns, beatings and official pleas for unity from the Supreme Leader is driving the regime towards increasingly extreme measures to enforce President Ahmadinejad’s hotly disputed re-election last Friday.

In an attempt to sway public opinion, the regime is blaming the protests on Western intelligence agencies. Last night it accused the United States — the “Great Satan” — of “intolerable” interference in Iranian affairs. The senior prosecutor in the province of Isfahan threatened demonstrators with execution, claiming that they were controlled by foreigners.

The regime could yet seek to crush the protests with a Tiananmen Square-style offensive, but the political elite is deeply split and there are said to be divisions even within the Republican Guard.

Grand Ayatollah Hossein Ali Montazeri, Iran’s most senior reformist cleric, said that vote rigging had undermined the Islamic regime’s legitimacy and “no sound mind” would accept the results.

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