US President Barack Obama says he is "deeply troubled" by violence in Iran following last week's disputed presidential elections.
Mr Obama called on Iran's leaders to respect free speech and the democratic process, and said he would continue pursuing tough dialogue with Iran.
His comments came after hundreds of thousands defied a government ban and attended an opposition rally in Tehran.
At least one person was killed after shots were fired during the protest.
In his first public comments on the situation in Iran, Mr Obama said: "It would be wrong for me to be silent on what we've seen on the television the last few days."
He added: "I think that the democratic process, free speech, the ability for folks to peacefully dissent, all those are universal values and need to be respected.
"Whenever I see violence perpetrated on people who are peacefully dissenting, and whenever the American people see that, I think they're rightfully troubled."
Mr Obama said it was for Iranians to decide who their leaders will be.
"We respect Iranian sovereignty and want to avoid the United States being the issue inside of Iran," Mr Obama said.
Opposition candidate Mir Hossein Mousavi has lodged a legal appeal against the election result but says he is not optimistic it will succeed.
He says the vote was fixed - a claim President Mahmoud Ahmadinejad denies.
On Monday, the European Union called on Iranian authorities to investigate allegations of fraud in the presidential elections.
EU foreign ministers said Iran had to address concerns expressed by several opposition candidates over how the vote was run.
Thousands join rally
