Washington  - The United States Monday said it has doubts about the results of Iran's election last week, which was declared in favour of President Mahmoud Ahmadinejad and has sparked days of violent protests.

"We are deeply troubled by the reports of violence, arrests and possible voting irregularities," said US State Department spokesman Ian Kelly.

"I think there has been doubt cast on the outcome of the elections," Kelly told reporters in Washington.

Hundreds of thousands protested in Tehran on Monday in support of opposition candidate Mir-Hossein Moussavi, who has demanded that the Iranian election be nullified.

The protest march went ahead despite being banned by Iran's Interior Ministry. At least two demonstrators were reportedly shot during the protest, but it was unclear who was to blame.

Ahmadinejad was declared the landslide winner of Iran's election last week, winning 62 per cent to Moussavi's 33.7 per cent of the vote.

Moussavi's supporters believe the vote was rigged. The country's top Guardian Council has reportedly agreed to investigate Moussavi's claims.

President Barack Obama, who has sought to engage Iran and ease tensions between the two powers, has been cautious not to weigh in too closely in favour of one side or the other. Kelly said the US and international community were "still assessing" the situation.

US Vice President Joe Biden on Sunday said the US found the election results "questionable" but would accept them "for the time being."  (dpa)