Iranian President Mahmoud Ahmadinejad has called on Barack Obama to apologise for alleged US interference after disputed elections in the Islamic republic.
Mr Ahmadinejad in Tehran celebrating his victory in the presidential elections
The hard-line leader also accused his American counterpart of behaving like his predecessor George Bush.
The UK has expelled two Iranian diplomats after Iran forced two British envoys to leave.
Gordon Brown told MPs: "Iran yesterday took the unjustified step of expelling two British diplomats over allegations which are absolutely without foundation.
"In response to that action, we informed the Iranian ambassador today that we would expel two Iranian diplomats from their embassy in London.
Iran's supreme leader has insisted there was no vote-rigging in a presidential election and attacked "enemies" - naming the UK government as the "most treacherous".
Anger after President Mahmoud Ahmadinejad was re-elected led to the biggest street demonstrations in the Islamic Republic's history.
Iran's opposition leader has reportedly called for a mass rally on Thursday in a direct challenge to the nation's Supreme Leader Ayatollah Ali Khamenei.
Defeated presidential candidate Mirhossein Mousavi is apparently urging his supporters to keep protesting over a disputed election and the alleged deaths of demonstrators.