President Obama has ruled out direct intervention in the electoral turmoil in Iran, warning that US attempts to interfere will backfire and that America has little to gain by throwing in its lot with the opposition.

But his public reluctance to interfere has been undermined by the actions of the State Department, which took the unusual step of asking Twitter to delay planned maintenance work so that Iranian protesters can continue to use it to post images and reports of unrest.

Mr Obama is facing calls from critics at home to take a much stronger international lead in condemning events in Iran, where the regime last night raided more universities.

But Mr Obama said that America had no long term interest in backing Mir Hossein Mousavi, the opposition candidate who is disputing the poll results that returned Mahmoud Ahmadinejad, Iran's right wing president, to power with a thumping – and, to some analysts, improbable – 63 per cent of the vote.

"It is not productive, given the history of US-Iranian relations, to be seen as meddling - the US president, meddling in Iranian elections," Mr Obama told CNBC television.

Two prominent reformists were arrested in Tehran this morning. Hamid Reza Jalaipour, a sociologist and Mousavi campaigner, was held at his home and Saeed Laylaz, a political and economic analyst, was also arrested by four plainclothes officials. Both are also prominent journalists.

Mousavi supporters are planning another mass demonstration in Tehran today, and protests have been reported in the cities of Shiraz, Tabriz, Isfahan, Meshad and Qom. Teargas was reportedly used at Kharazmi university in Shiraz, with students beaten up and 100 arrests. The head of the university resigned. Riot police also stormed the university in Isfahan.

The Iranian authorities have tried hard to prevent images of the turmoil being portrayed at home and abroad. Foreign journalists have been banned from the streets, and ordered to compile their stories by telephone from the offices. Reporters' visas have not been renewed.

Twitter has gained in importance since internet sites, mobile phone and SMS texting networks have all been intermittently blocked by the authorities in a cat-and-mouse game with the demonstrators who have used them to organise more protests.

Today the Iranian Revolutionary Guard, one of the most powerful organisations in Iran, broke its silence for the first time since the elections with an ominous warning to Iranian websites and bloggers to remove any seditious materials or face legal action.

In Tehran the Iranian Foreign Ministry summoned Simon Gass, the British Ambassador, to complain about Gordon Brown’s “unconventional and impolite” remarks challenging the election’s legitimacy.

The Government also arrested what the state media described as the “main agents” of the unrest. Gholamhossein Mohseni-Ejei, the Intelligence Minister, said that about 50 people with weapons and explosives were detained, and claimed that they were supported by foreign powers. Two dozen “counter-revolutionaries” were also arrested, as were two leading reformists, including Ali Abtahi, a former Vice-President.

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