Iran has arrested opposition leaders Mir Hossein Mousavi and Mehdi Karroubi and has taken them, along with their wives, to a prison operated by the Revolutionary Guards in Tehran, their family members said Monday.

The family members said that neighbors told them of the arrests and that they had occurred on Thursday afternoon. They also said Iranian security agents made the arrests without a judicial process and without an arrest warrant.

The Wall Street Journal cited advisers to the opposition leaders, who said a senior commander of the Revolutionary Guards confirmed the news of the arrests and said the two were taken to the Heshmatieyh prison, a high-level security facility inside a Revolutionary Guard military base in eastern Tehran.

The report of the arrests came just one day before a planned “Tuesday of Protest”, a string of anti-government demonstrations which the opposition had called for. In response to the reports, furious opposition supporters posted messages on Facebook and Persian websites and blogs pledging to take to the streets Tuesday.

“The regime crossed the red line. Tomorrow will be our turn to rise up against them,” read a post on the Green Movement's Facebook page.

The Iranian government would not confirm the arrests. The Fars News Agency quoted an anonymous judiciary official who denied that Mousavi and Karroubi were arrested and said they were at their homes.

“A judiciary source has denied the arrest of the chiefs of sedition Mousavi and Karroubi,” said the report in Fars. “They are currently in their homes and are faced only with restrictions to contact suspect elements.”

Mousavi’s website, Kalame, claimed that Supreme Leader Ayatollah Ali Khamenei ordered the two arrested following the anti-government protests on February 14 and 20 which called for his ouster.

Mousavi and Karroubi had been placed under house arrests following those protests. Following the house arrest, members of Iran’s parliament called to kill the two. “Moussavi, Karrubi... execute them,” they called.

United States President Barack Obama has expressed support for protests in Iran and has called for Middle East leaders to listen to protesters’ messages. He has also criticized Iranian leaders for brutally suppressing demonstrations in their own country while praising protesters in Egypt.

Meanwhile on Monday, prosecutor general Gholam Hossein Mohseni Ejeie warned the opposition movement against plans to hold protests on Tuesday.

“Those who are working against the Islamic regime and spreading rumors abroad, whether they are Iranian or foreign, should know that they will be hunted down,” said Ejeie. His comments added further confusion over whether Mousavi and Karroubi had indeed been arrested, AFP reported.

Mousavi and Karroubi lost to President Mahmoud Ahmadinejad in the June 2009 election and have since maintained that the elections were fraudulent. They led a string of protests, some of which had become violent, after Ahmadinejad's re-election.

The United States responded to reports of the arrests and said on Monday that they were unacceptable.

“We obviously find the detention of opposition leaders to be unacceptable,” said White House Spokesman Jay Carney. “And we call (for) them to be treated well and released.”

Carney added that “We note with continued astonishment the hypocrisy of the Iranian government.”