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.

Yazdi was the foreign minister in Iran's first government after the 1979 revolution but has been sidelined since hardliners took control.

Mehdi Noorbaksh, Yazdi’s son-in-law who lives in Harrisburg, Pa., confirmed that he was arrested at Pars Hospital at around 3 p.m. 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 Yazdi was arrested in the intensive care unit.

So far 200 prominent journalists, opposition politicians and reformists have been rounded up by the regime, 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 Web site.

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Supporters of Iranian opposition leader Mir Hossein Mousavi took to the streets of Tehran Thursday for a fourth day of protests over the hotly-disputed presidential election, a witness said.

Mousavi called his followers back into the streets of Tehran for another mass rally Thursday, urging them to wear black to mourn those killed in clashes. Mousavi claims Friday's election was rigged in favor of President Mahmoud Ahmadinejad.

Meanwhile, state radio reported that Iran's Guardian Council, an unelected body of 12 clerics and Islamic law experts close to Supreme Leader Ayatollah Ali Khamenei, was to meet Saturday with election candidates to discuss complaints about the election, according to Reuters.

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Mousavi, who claims the election was rigged and he was the true winner, alleges that the Guardian Council is not neutral and has already indicated it supports Ahmadinejad. He wants an independent investigation.

Violent clashes erupted after the election commission announced Ahmadinejad had won in a landslide. Seven demonstrators were shot Monday by pro-regime militia in the first confirmed deaths during the unrest.

Thursday's protest would be the fourth straight day of major marches in Tehran. On Monday, hundreds of thousands turned out in a huge procession that recalled the scale of protests during the 1979 Islamic Revolution. Mousavi's Web site said he may join the rally on Thursday in downtown Tehran.

On Wednesday, thousands marched silently down a main street in the capital, holding posters of Mousavi and flashing the V-for-victory sign in the air, amateur video showed.

The street protests have presented one of the gravest threats to Iran's complex blend of democracy and religious authority since the system emerged out of the 1979 Islamic revolution.

Any serious shift of the protest anger toward Iran's non-elected theocracy would sharply change the stakes. Instead of a clash over the June 12 election results, it would become a showdown over the core premise of Iran's system of rule — the almost unlimited authority of the clerics at the top.

On Thursday, state radio reported that the council has invited Mousavi and two other candidates who ran against Ahmadinejad to a meeting in the coming days.

The Iranian government accused the United States on Wednesday of meddling in the deepening crisis. State media blamed Washington for "intolerable" interference.

"Despite wide coverage of unrest, foreign media have not been able to provide any evidence on a single violation in the election process," state radio said Thursday.

Authorities have rounded up perceived dissidents and tried to further muzzle Web sites and other networks used by Mousavi's backers to share information and send out details of the crisis after foreign journalists were banned from reporting in the streets.

President Obama said he shared the world's "deep concerns" but it was "not productive, given the history of U.S.-Iranian relations, to be seen as meddling." The two countries severed diplomatic relations after the 1979 Islamic Revolution in the bloody showdown over allegations of vote-rigging and fraud.

A crackdown on dissent continued, with more arrests of opposition figures reported, and the country's most powerful military force — the Revolutionary Guard — saying that Iranian Web sites and bloggers must remove materials that "create tension" or face legal action.

The government has blocked certain Web sites, such as BBC Farsi, Facebook, Twitter and several pro-Mousavi sites that are vital conduits for Iranians to tell the world about protests and violence. Many other sites, including G-mail and Yahoo, were unusually slow and rarely connect.

Mousavi has condemned the blocking of Web sites, saying the government did not tolerate the voice of the opposition.

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On Thursday, Mousavi's Web site said that both Mousavi and former reformist President Mohammad Khatami sent a joint letter to Iran's head of judiciary, Ayatollah Mahmoud Hashemi Shahroudi, asking him to take measures to stop violence against protesters by police and help to release detained demonstrators.

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