France, Germany and Britain led EU calls on Iran to clarify the result, as sporadic shooting broke out in Tehran after big demonstrations by supporters of moderate candidate Mirhossein Mousavi.
French President Nicolas Sarkozy said in a statement that any manipulation of the election would be unacceptable.
Sarkozy "condemns violence against demonstrators, arrests of opponents and of politicians, restrictions of public freedoms, freedom of speech and of communication, and obstacles to the work of foreign and Iranian journalists," the statement said.
In Washington, where President Barack Obama has promised to reach out to try to end 30 years of hostility with Tehran, the administration stressed it was waiting for the outcome of the post-election wrangling before making a definitive comment.
"We are deeply troubled by the reports of violence, arrests and possible voting irregularities," State Department spokesman Ian Kelly said, adding Washington was still assessing what had happened in the election.
"We are calling for Iran's authorities to respect the right of people to express themselves peacefully," he said.
Kelly said serious doubts had been cast on the official result, which gave incumbent Ahmadinejad a resounding victory. Mousavi wants the result of Friday's election annulled.
"The authorities in Iran need to take these concerns seriously. They need to look into them. But I'm not going to stand here today and predict how we are going to judge, whatever the outcome is," said Kelly.
"We're hamstrung by the fact that Iran didn't allow international monitors. We don't have diplomats on the ground," he said.
Iran’s nuclear programme
European countries have been working with the United States to try to persuade Tehran not to develop nuclear weapons.
Iran's Guardian Council, whose chairman, Ayatollah Ahmad Jannati, endorsed Ahmadinejad before the vote, said it would rule within 10 days on two official complaints it had received from Mousavi and another losing candidate, Mohsen Rezaie. The council must approve results for the outcome to stand.
German Chancellor Angela Merkel told a news conference in Berlin that "there are signs of irregularities". Government spokesman Thomas Steg told reporters there must be a quick examination of the allegations of manipulation.
"We are greatly concerned about what we see as an over-reaction by the security forces in cracking down on protesters, people who have the right to express their opinions," Steg said.
Ahmadinejad and Interior Ministry officials have dismissed allegations the vote was rigged.
The outcome has alarmed Western powers trying to persuade the world's fifth-biggest oil exporter to curb nuclear work they suspect is intended to develop atomic weapons, a charge Iran denies.
British Prime Minister Gordon Brown said Iran must address questions raised about the election and that its response to protests would affect its relations with other countries. Speaking in parliament, he said "the way the regime responds to legitimate protests will have implications for Iran's relationships with the rest of the world in future". He did not elaborate.
Daniel Rackowski, an analyst with the Transatlantic Institute in Brussels, criticised the US approach. "When it comes to Iran, the United States have recently stuck their necks out and are being overly cautious," he said.
"The United States probably know they will have to work with Ahmadinejad and they will be the ones doing the bulk of the negotiations, not the European Union. So I think the European Union can be more critical whereas [US Secretary of State Hillary] Clinton will actually have to work with them."
(EurActiv with Reuters.)
