Mr Mousavi had become the protesters' representative, Barack Obama said
US President Barack Obama has praised the opposition candidate in Iran's disputed presidential election.
Mr Obama said Mir Hossein Mousavi had captured the imagination of groups in Iran that were interested in opening up to the world.
He spoke of the bravery of protesters in the face of "outrageous" violence.
Mr Obama's comments came hours after foreign ministers from the G8 nations issued a statement "deploring" the post-election violence in Iran.
In Iran itself, a spokesman for the powerful Guardian Council - which is due to give its final ruling on the election on Sunday - said there had been no election fraud.
And a member of Iran's top clerical body urged the judiciary to deal ruthlessly with the leaders of the protests.
Some 17 people are thought to have died in street protests in the past two weeks, and Tehran has imposed severe restrictions on journalists and the internet.
Rights group Amnesty International called on Iranian leaders to release more than two dozen journalists arrested since the polls.
Mr Obama made his comments at a news conference in Washington after talks with German Chancellor Angela Merkel.
Mr Mousavi had become the representative of protesters on the streets who, he said, had displayed "extraordinary courage".
"The violence perpetrated against them is outrageous," Mr Obama said. "In spite of the government's efforts to keep the world from bearing witness to that violence, we see it and we condemn it."
In Italy, meanwhile, G8 foreign ministers said they respected the sovereignty of Iran, but deplored the post-electoral violence.
"We express our solidarity with those who have suffered repression while peacefully demonstrating and urge Iran to respect fundamental human rights," their statement said.
"The crisis should be settled soon through democratic dialogue and peaceful means."
But the G8 leaders said the door to dialogue with Iran must remain open. And the G8's comments were not as strong as France and Italy had wanted, after Russia warned against isolating Iran.
Russian Foreign Minister Sergei Lavrov said engagement with Iran was "the key word".
He stressed the need to focus on "the main task - to move toward resolving the issues of the Iranian nuclear programme".
BBC News, Tehran
The Guardian Council is due to give its definitive verdict on Sunday.
But the remarks by its spokesman are yet another indication that it will be a formality.
The question though is whether the fracture in the ruling elite that this crisis has caused will heal.
When you ask Iranians about the way this might go, a phrase keeps cropping up. They say it might seem quiet to an outsider but there is fire below the ashes.
Before the G8 issued its statement, a spokesman for Iran's top election body, the Guardian Council, said the vote had been fair.
"We have had no fraud in any presidential election and this one was the cleanest election we have had," Abbasali Kadkhodai told Irna news agency.
Meanwhile, a senior hard-line cleric said in his Friday sermon that the leaders of the protests should be dealt with "severely and ruthlessly".
"I want the judiciary to... punish leading rioters firmly and without showing any mercy to teach everyone a lesson," Ahmad Khatami told worshippers at Tehran university in comments broadcast nationwide.