The most prominent figure pushing back on Khamenei has been Mousavi, a pro-reform centrist and former prime minister who ran for president and claimed he beat Ahmedinejad. Mousavi has become the figurehead of the opposition movement, rallying a broad mass of supporters and showing up at Saturday's protests despite heavy restrictions on his movement.
Mousavi attended Saturday's chaotic rally in Tehran, vowing to challenge the official election results, instructing supporters to go on strike if he was arrested and expressing his willingness to die for his cause.
"I am prepared for martyrdom," Mousavi said on his Twitter and Facebook pages.
President Barack Obama spoke out on the events, telling the Iranian government "the world is watching."
"We mourn each and every innocent life that is lost," Obama said in a written statement. "We call on the Iranian government to stop all violent and unjust actions against its own people."
The election and its aftermath have complicated Obama's efforts at new diplomacy between the United States and Iran, which would break 30 years of strained ties and antagonism.
Supporters of Mir-Hossein Mousavi have called on the United States and other countries not to recognize Ahmedinejad's election win.
Obama has said outreach efforts would continue, though it would be difficult to do so without at least tacitly acknowledging Ahmedinejad's second term.