Tehran University looked as calm as any summer campus. So much for the latest rumours of a bloodbath. Another piece of Iranian fiction, served up on YouTube. Scarved female students were moving through the university's great black iron gates. I asked my driver, Ali, to drop me off at the corner so I could prowl the college bookshops on Engelob Street, I was looking for a volume of modern Persian poetry for a friend. I did not at first hear the man at the cash desk, motioning out the door.

I peered out. The gates of the university were now shut. Behind them was a crowd of hundreds of young men and women, many wearing scarves over their mouths. I crossed the road. And the banners behind those forbidding gates told a frightening story. "Today is a day of mourning," one of them read. "Dignified students are mourners today." "Police, shame on you, shame on you." "Tell my mother – she doesn't have a son any more."

I walked up to the gate. Young female students were crying. So were some of the young men. "We don't want a government by coup," another poster read. "Tehran University dormitory has been coloured with students' blood," another said.

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It was difficult to hear over the cries and screaming. But a student began shouting at me in English through those grim black gates. "There was a massacre," he bellowed. "The Basiji and the police came into our student dorms. It all started after the violence last Saturday. The people in the street had been throwing stones, so many of us fled from the campus to our homes. We came back yesterday and it seemed quiet. Then all these armed men burst into the dorms, shooting."

One girl spoke of five dead, another of seven. A student suggested the dead men were not students. Were they hiding on campus? It wasn't clear. Within hours, photographs of blood appeared on the internet. Who were these mysterious victims – for dead men there surely were. The crowds began to run in panic and behind them I spotted the familiar glint of steel helmets. I've now learned how to deal with these gentlemen. You never, ever run. You saunter towards them and if a single one moves his baton towards you, you click your finger so that he thinks that you have a right to be there. Then you stand just behind them, nodding in a friendly way when they look at you.

One of the cops turned round with a cynical smile. "Welcome to our country," he said. A couple of officers waved me away but I waved back my press card and they lost interest.

Did these cops know what had happened here? Did they have any idea how much these students hated them? A big plain-clothes man walked up and pointed his finger across the road. More of the same kind were waiting on the other pavement. "Papers?" one asked. He spent five minutes staring at my press card. Behind him I could see the cops had climbed into the campus.

Two had seized a young man, struggling between them, terrified, before the first baton came down on his head. I didn't hear the crack as the stick hit the student.

My driver was petrified. He has no journalistic papers. He had to be protected. So we left. As usual, the SMS system was down, the mobile phones were cut, the internet took half an hour to send a single message. No calls to London or New York or Paris... Whenever the communications collapse here, you know that something is afoot. Could it be that the police know when they are doing something wrong?

Campus power: Student demonstrations hold key

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By Adrian Hamilton

*A co-ordinated series of demonstrations by students in all the major cities of Iran throughout 1978 were instrumental in bringing down the Shah in early 1979.

It is difficult to know whether they will be able to keep up their resistance this time, but their position could prove pivotal. Iran is a young country with half its population under 25. It is also unique in its proportion of women in higher education, at 60 per cent.

For the last four years, it has been Iran's campuses that have raised the standard of rebellion, demanding freedom of expression and relief from conservative rules on dress and behaviour, only to be put down with increasing severity and considerable bloodshed. Demonstrations were held in 20 centres last summer, while Tehran students held a series of protests directly attacking President Ahmadinejad as a "tyrant" last autumn in which there were several deaths.

The protests have already started again in Tehran's huge campus, but the authorities will be especially concerned if this starts spreading – as it seemed to be doing last summer – at which time Isfahan and Ahwaz were particular hotspots.

If rumours of student deaths and violent suppression start spreading, an uprising could be difficult to control.