Ayatollah Ali Khamenei is ordering the powerful Guardian Council to examine the allegations by pro-reform candidate Mir Hossein Mousavi, who claims widespread vote rigging in Friday's election. The government declared President Mahmoud Ahmadinejad the winner in a landslide victory.
It is a stunning turnaround for Iran's most powerful figure, who previously welcomed the results.
Mousavi wrote an appeal Sunday to the Guardian Council, a powerful 12-member body that's a pillar of Iran's theocracy. Mousavi also met Sunday with Khamenei.
Mousavi's backers have waged three days of street protests in Tehran.
State televiosn further reported that Khamenei told Mousavi on Sunday to pursue his complaints over the election through legal means and settle the issue calmly.