Mousavi and Karroubi, who rose to prominence as "Green Movement" candidates in the disputed 2009 elections, had called Monday's rallies in Tehran and elsewhere to show solidarity with recent Arab uprisings against authoritarian governments.
In countries across the Middle East and in the Persian Gulf, citizens long angry with autocratic regimes, incessant corruption and stagnant economies took to the streets Monday demanding change.
But unlike in Egypt and Tunisia, where the armies vowed to not fire on demonstrators, other governments have shown a willingness to crush all opposition — sometimes with deadly force.
In Bahrain, police fired rubber bullets and tear gas at protesters. The New York Times reported that the police fired so much tear gas, their own officers were left vomiting.
In Yemen, student protesters took to the streets for a fourth day, demanding the president's resignation. Security forces have been using electric tasers to attack the protesters, according to Human Rights Watch.
Algeria saw recent protests that also looked to be inspired by the revolutions in Tunisia and Egypt. The government unleashed thousands of riot police to prevent the protests from getting too large.
Meanwhile in Egypt, where protests forced President Hosni Mubarak to step down on Friday, members of the country's internal security forces marched Sunday and Monday to demand better pay and represent themselves as the nation's victims.
In Iran, one person has been killed during clashes between security forces and protesters in the capital Tehran. Police blamed the outlawed People's Mujahedeen of Iran (PMOI) for shooting into the crowd, although the National Council of Resistance of Iran, which includes the PMOI, denied the allegations on Tuesday, saying "those in power crushed the demonstrators, firing live rounds and tear gas at them."
Hundreds of riot police beat protesters and fired tear gas Monday at the tens of thousands of protesters who emerged on the streets. The Associated Press quoted witnesses saying that at least three protesters had been injured by bullets while dozens more were hospitalized after being beaten.
Mousavi's website carried "unconfirmed reports" that security forces had arrested hundreds of people during the demonstrations, which drew thousands of activists to the streets of the capital in defiance of a government ban.
Conservative lawmakers said Tuesday that he should be tried for sedition, an offense punishable by death, along with Karroubi.
The demonstrations in Iran were the largest since the 2009 uprising over Mahmoud Ahmadinejad's reelection to the presidency in disputed circumstances.
Activists who marched in Tehran on Monday also chanted "death to the dictator" — a slogan used by reformists who protested the re-election of conservative Ahmadinejad in a disputed 2009 vote. By late Monday, similar chants echoed from Tehran rooftops, as they did during the unrest two years ago.
The Iranian parliamentary speaker Ali Larijani, meanwhile, accused the United States and its allies of using Iran's opposition movement as a tool to incite unrest, according to VOA.