Iran has banned foreign journalists from leaving their offices to cover street protests over the hotly-contested presidential poll. The election was won by hardline incumbent Mahmoud Ahmadinejad, prompting defeated candidate Mir Hossein Mousavi to criticise the result and claim widespread fraud. The country is witnessing the biggest demonstrations since the 1979 Islamic revolution. State television has said the "main agents" in the unrest have been arrested with explosives and guns but gave no further details. A Culture Ministry official said of the media clampdown: "No journalist has permission to report or film or take pictures in the city." Meanwhile, Iran's top legislative body, the 12-man Guardian Council, has refused to annul the poll but says it is "ready to recount the disputed ballot boxes claimed by some candidates, in the presence of their representatives". Seven people are reported to have been killed on the fringes of a vast demonstration held in the captial Tehran on Monday in protest at the election result and there have been widespread arrests across the country. An Iranian photographer at the scene said Islamic militiamen opened fire when people in the crowd attacked a post of the Basij religious militia - a volunteer paramilitary force fiercely loyal to Supreme Leader Ayatollah Ali Khamenei, who has the final say on all matters of state. Mr Mousavi's outraged supporters are hoping to hold another big rally but instead, state TV has aired pictures of thousands of President Ahmadinejad supporters, some waving Iranian flags, gathering at the Vali-ye Asr Square. Mr Mousavi has told his followers to stay away from the square "to protect lives" and avoid possible confrontation with security forces and backers of Mr Ahmadinejad. US President Barack Obama, who has sought to engage Iran and asked its leadership to "unclench its fist", said he is deeply troubled by the violence and that protesters who had taken to the streets had inspired the world. The US and its European allies have been trying to engage Iran and induce the world's fifth-biggest oil exporter to halt nuclear work that could be used to make an atomic bomb. Iran says it only wants nuclear energy to generate electricity. Mr Obama said he would continue pursuing tough, direct dialogue with Tehran but urged that any Iranian investigation of election irregularities be conducted without bloodshed. The world was inspired by the Iranian protesters, he said.