15:40 GMT TEHRAN
BBC CORRESPONDENT

The police couldn't be everywhere and halfway along the avenue leading from one of the squares to the other, I watched a crowd slowly gather and begin chanting, setting fire to rubbish bins and throwing stones at the police.

They had achieved the critical mass they needed and more and more demonstrators joined them. Near me in the crowd a man was shot in the arm and the air was thick with tear gas. I saw another man whose arm had been slashed by a razor wielded by a secret policeman.

The confrontations are still going on and a big column of black smoke from a fire is hanging over the city centre. - BBC -