It happened in a trice. Chris Harris, the Times photographer, and I were driving around Vali Asr Square in the centre of Tehran when a crowd of Mousavi supporters coalesced in one corner and started chanting. In no time they were charged by a posse of helmeted riot police who lashed out with their batons at men, women — anyone in sight.

We jumped from our fixer’s car and Chris instinctively started taking photographs. Two of the riot police made a bee-line for him. They grabbed him round the neck and frogmarched him away. There was no way they wanted the world to see pictures of their brutality. It took 30 minutes to find my fixer in the chaos and another 30 minutes to discover where Chris had been taken.

We found him in a police station. The officer guarding him was mortified. “Ahmadinejad bribed his way to victory . . . It’s really shameful. I don’t know what to say about it,” he said.

After two hours Chris, with our fixer, was transferred to the police information branch in north Tehran. Satisfied he was safe, I took a taxi to Mr Mousavi’s campaign headquarters in a side road off Vali Asr Street. It was sealed off and knots of angry Mousavi supporters were milling around. Suddenly everyone started running. Scores of riot police were charging, many of them on motorcycles, batons flailing. I felt the lash of a truncheon across my left shoulder as one roared past.

I ducked into a sidestreet where an Iranian journalist was also taking refuge. I asked her to translate a flyer put out by the Mousavi camp. As she did so, a burly man in plain clothes walked up, snatched the flyer and read it. He demanded to see our press credentials, then marched us into the nearby Ministry of Alien Affairs. A more senior officer was summoned. It took 30 minutes before the Iranian journalist persuaded them to let us go.

Chris and I arrived back at our hotel almost simultaneously. He had been released after deleting his photographs of the riot police, though he later retrieved them.

Scores of foreign journalists were picked up as they recorded the ruthless suppression of Mr Mousavi’s supporters. Almost all were released, but many had their cameras confiscated and pictures erased. By the middle of this week almost all foreign journalists will have been forced to leave Iran, as the regime has refused to extend the ten-day visas they were granted to cover the election.

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