The Obama administration on Sunday strongly condemned the, offering its support to civilians "seeking to exercise their universal rights."
National Security Council spokesman Mike Hammer on Sunday denounced Tehran's "unjust suppression of civilians" in a crackdown that has killed at least five people, including a nephew of opposition leader Mir Hossein Mousavi.
"Governing through fear and violence is never just," Hammer said.
Hammer quoted President Barack Obama's Nobel Peace Prize acceptance speech, saying "it is telling when governments fear the aspirations of their own people more than the power of any other nation."
Witnesses and opposition Web sites said Iranian security forces fired on stone-throwing protesters in the center of Iran's capital Sunday.
The protests began with thousands of opposition supporters chanting "Death to the dictator," a reference to hard-line President Mahmoud Ahmadinejad, as they marched in defiance of official warnings of a harsh crackdown on any demonstrations coinciding with Shiite Islam's most important observance, Ashoura. The observance commemorates the seventh-century death in battle of one of Shiite Islam's most beloved saints.
Security forces tried but failed to disperse protesters on a central Tehran street with tear gas, baton charges and warning shots. They then opened fire on protesters, said witnesses and the Rah-e-Sabz Web site.