By Parisa Hafezi and Fredrik Dahl
TEHRAN (Reuters) - Thousands of supporters of Iran's defeated presidential candidate Mirhossein Mousavi marched in Tehran on Tuesday following a disputed poll prompting the biggest street protests since the 1979 Islamic revolution.
(Editor's note: Reuters coverage is now subject to an Iranian ban on foreign media leaving the office to report, film or take pictures in Tehran.)
On a fourth day of demonstrations since Friday's election, witnesses said they headed -- largely in silence without the accustomed chants -- towards the state television building, despite Mousavi's call for them to call off a planned rally.
In what appeared to be a first concession by authorities to the protest movement in the world's fifth-biggest oil exporter, Iran's top legislative body said it was prepared for a partial recount but ruled out annulling the poll.
The decision was taken by the 12-man Guardian Council following the election in which hardline President Mahmoud Ahmadinejad was declared the runaway winner.
In an apparent attempt to deny the opposition the chance to keep up the momentum of mass street protests, Ahmadinejad's supporters mobilised thousands of demonstrators in central Tehran where Mousavi's supporters had planned to gather again.
The Mousavi supporters marching on Tuesday said they planned to assemble in front of the state television IRIB building in northern Tehran. Witnesses said some supporters had already gathered close to the building, which was ringed by riot police.
Wearing wristbands and ribbons in his green campaign colours, Mousavi supporters carried his picture and made victory signs. Some were sending messages to others to meet again on Wednesday for a rally at Tehran's central Haft-e Tir Square.
As a column of riot police moved up the capital's most famous boulevard, tree-lined Vali-ye Asr Avenue, demonstrators clapped their hands in defiance. Traffic was blocked on the avenue and at one point about 100 protesters sat in the road.
Unlike Monday's vast opposition march in central Tehran, in which seven people were killed, the demonstrators largely refrained from chanting anti-Ahmadinejad slogans.
Later, witnesses heard shouts of support for Mousavi, cries of "Allahu akbar" (God is Great) and the blaring of car horns.
Mousavi has disputed official results of the poll. State television said the "main agents" in post-election unrest had been arrested with explosives and guns.
U.S. President Barack Obama, who has sought to engage Iran and asked its leadership to "unclench its fist," reacted on Tuesday to Iran's post-election violence by saying he believed "people's voices should be heard and not suppressed."
Obama also said he did not want to be seen as "meddling" in Iranian internal affairs.
The U.S. State Department said it had contacted the social networking service Twitter to urge it to delay a planned upgrade that would have cut daytime service to Iranians. Twitter and Facebook have been used as a tool by many young people to coordinate protests.
French President Nicolas Sarkozy said the images of repression of protesters were "shocking" and "unacceptable."
"The size of the fraud is proportional to the size of the reaction" of Iranian authorities, he said in the African state of Gabon, where he was marking the death of President Omar Bongo. "The Iranian people deserve something else."
The United States and its European allies have found Ahmadinejad implacable in asserting Iran's right to enrich uranium, a programme that Iran says is purely peaceful but that they fear could be used to make a nuclear bomb.
BAN ON RALLY
Iranian authorities had banned Tuesday's opposition rally and state television showed live pictures of thousands of Ahmadinejad supporters, some waving Iranian flags, gathering at Vali-ye Asr Square where Mousavi supporters had planned to go.
At the rally, a former parliamentary speaker, Gholamali Haddadadel, drew cheers by saying that Tehran, where Mousavi won the most votes, did not represent all of Iran. He added:
"I would like to tell Mr Mousavi:... Before the election was held it was not right for people close to you to say that if you see Mr Ahmadinejad victorious there has been electoral fraud."
Mousavi had urged his supporters to stay away from the square "to protect lives."
Further protests, especially if they are on the same scale as Monday's, would be a direct challenge to the authorities who have kept a tight grip on dissent since the U.S.-backed shah was overthrown in 1979 after months of demonstrations.
Discord within Iran's ruling system has never been so public. The Mousavi camp is backed by traditional establishment figures, such as former presidents Akbar Hashemi Rafsanjani and Mohammad Khatami, concerned about how Ahmadinejad's truculent foreign policy and populist economics are shaping Iran's future.
Supreme Leader Ayatollah Ali Khamenei, supposedly above the political fray, has favoured Ahmadinejad, who is also supported by the elite Revolutionary Guard and the Basij militia.
Intelligence Minister Gholamhossein Mohseni-Ejei said his ministry was chasing two types of people over the unrest.
"One wanted to achieve its goal through explosions and terror, and in this connection 50 people were arrested and more than 20 explosive consignments were discovered. They were supported from outside the country," he told state radio.
"The second category was made up of counter-revolutionary groups who had penetrated election headquarters (of the election candidates) ... Some 26 such elements have been arrested," he added. Iran often accuses Western foes of stirring instability.
Illustrating Iran's sensitivity to world opinion, authorities on Tuesday banned foreign journalists from leaving their offices to cover street protests.
A spokesman for the Guardian Council, which groups clerics and Islamic law experts as a constitutional watchdog, said that it was "ready to recount the disputed ballot boxes claimed by some candidates, in the presence of their representatives."
"It is possible that there may be some changes in the tally after the recount," spokesman Abbasali Kadkhodai said. "Based on the law, the demand of those candidates for the cancellation of the vote - this cannot be considered," he told state television.
(Writing by Charles Dick)
