World is watching, Obama warns Iran

Mr Obama spoke out after Iran's supreme leader Ayatollah Ali Khamenei demanded that opposition supporters behind the mass protests end their demonstrations - and warned protest leaders would be held responsible for any "bloodshed" if his demand is not met.

"I'm very concerned based on some of the tenor and tone of the statements that have been made," Mr Obama warned.

He added: "We stand behind those who are seeking justice in a peaceful way. Wee stand with those who would look to peaceful resolution of conflict and we believe that the voices of people have to be heard."

Despite the ayatollah's warning, more demonstrations are expected on the streets of Tehran to again protest against alleged vote rigging, leading to fears stern action may be taken by security officials to stamp them out.

Meanwhile, the country's Guardian Council has summoned opposition leader Mir Hossein Mousavi and two other candidates who stood against President Mahmoud Ahmadinejad in last week's election, for talks.

The council says it wants to discuss 600 complaints the trio have made which allege vote rigging.

Mr Mousavi and his backers are demanding a re-run of the election, claiming he was the successful candidate and not Mr Ahmadinejad.