President Barack Obama challenged Iran's government to halt a "violent and unjust" crackdown on dissenters, using his bluntest language yet to condemn Tehran's postelection response.
Mr Obama has sought a measured reaction to avoid being drawn in as a meddler in Iranian affairs.
Yet his comments have grown more pointed as the clashes intensified, and his latest remarks took direct aim at Iranian leaders.
"We call on the Iranian government to stop all violent and unjust actions against its own people," Mr Obama said in a written statement.
"The universal rights to assembly and free speech must be respected, and the United States stands with all who seek to exercise those rights."
Mr Obama has searched for the right tone in light of political pressures on all sides. On Capitol Hill, Congress pressed him to condemn the Iranian government's response.
In Iran, the leadership was poised to blame the US for interference and draw Mr Obama in more directly.
Mr Obama met with advisers at the White House as developments in Iran grew more ominous, with police seen beating and clashing with protesters.
"Suppressing ideas never succeeds in making them go away," the president said, recalling a theme from the speech he gave in Cairo, Egypt, earlier this month.
"The Iranian people will ultimately judge the actions of their own government," Mr Obama said. "If the Iranian government seeks the respect of the international community, it must respect the dignity of its own people and govern through consent, not coercion."
Protesters in Iran have demanded that the government cancel and rerun the June 12 elections that ended with a declaration of overwhelming victory for hardline President Mahmoud Ahmadinejad.
Reformist presidential candidate Mir Hossein Mousavi says he won and claimed widespread fraud.
Iran's supreme leader, Ayatollah Ali Khamenei, said there was no ballot rigging. He warned on Friday of a crackdown if protesters continued their massive street rallies.
Then yesterday, police beat protesters and fired tear gas and water cannons at thousands who rallied in open defiance of Iran's clerical government.
Mr Obama cited civil rights leader Martin Luther King Jr's statement that "the arc of the moral universe is long, but it bends toward justice".
"I believe that," the president said. "The international community believes that. And right now, we are bearing witness to the Iranian people's belief in that truth, and we will continue to bear witness."