US President Barack Obama is resisting pressure to side with Iran's opposition as mass protests continue over the nation's disputed presidential poll.
In a TV interview Mr Obama said there might not be much difference between the policies of President Ahmadinejad and rival Mir Hossein Mousavi.
Mr Mousavi's supporters have continued street protests despite the threat of government force and earlier bloodshed.
BBC correspondents in Tehran say the mood in the city is tense and angry.
Tough new restrictions have been imposed on foreign media in Iran.
But state TV showed a rally by government supporters in the centre of Tehran. It did not appear as big or as enthusiastic as the yesterday's massive opposition protest.
The government appears divided over how to respond. The opposition enjoys huge and vocal support, but is not well organised.
It is a deadlock shaking the very foundations of the Islamic Republic, and there is no clear picture what is going to happen next.
But despite government attempts to control the flow of information out of the country, Iranians have been using the internet to send images and personal accounts of the protests around the world.
Mr Obama said he believed Iranian voices should be heard, although he added that he did not want to be seen to be "meddling".
/*/ "It is not productive, given the history of US and Iranian relations to be seen as meddling in Iranian elections," he said. /*/
"But when I see violence directed at peaceful protesters, when I see peaceful dissent being suppressed… it is of concern to me and it is of concern to the American people."
***** Speaking later in the television interview, he downplayed the importance to the world of the struggle for power in Iran.
"The difference between Ahmadinejad and Mousavi in terms of their actual policies may not be as great as has been advertised," he said. *****
Earlier on Tuesday, the US state department said it had asked the social networking site Twitter to delay maintenance work so that Iranians could keep using it as a communications tool.