At around 4.35 last Monday morning, my Beirut mobile phone rang in my Tehran hotel room. "Mr Fisk, I am a computer science student in Lebanon. I have just heard that students are being massacred in their dorms at Tehran University. Do you know about this?" The Fisk notebook is lifted wearily from the bedside table. "And can you tell me why," he continued, "the BBC and other media are not reporting that the Iranian authorities have closed down SMS calls and local mobile phones and have shut down the internet in Tehran? I am learning what is happening only from Twitters and Facebook."

When I arrived at the university, the students were shrieking abuse through the iron gates of the campus. "Massacre, massacre," they cried. Gunfire in the dorms. Correct. Blood on the floor. Correct. Seven dead? Ten dead, one student told me through the fence. We don't know. The cops arrived minutes later amid a shower of stones. Filtering truth out of Tehran these days is as frustrating as it is dangerous.

A day earlier, an Iranian woman muttered to me in an office lift that the first fatality of the street violence was a young student. Was she sure, I asked? "Yes," she said. "I have seen the photograph of his body. It is terrible." I never saw her again. Nor the photograph. Nor had anyone seen the body. It was a fantasy. Earnest reporters check this out – in fact, I have been spending at least a third of my working days in Tehran this past week not reporting what might prove to be true but disproving what is clearly untrue.

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Take the call I had five hours before the early-hour phone call, from a radio station in California. Could I describe the street fighting I was witnessing at that moment? Now, it happened that I was standing on the roof of the al-Jazeera office in north Tehran, speaking in a late-night live interview with the Qatar television station. I could indeed describe the scene to California. What I could see were teenagers on motorcycles, whooping with delight as they set light to the contents of a litter bin on the corner of the highway.

Two policemen ran up to them with night-sticks and they raced away on their bikes with shouts of derision. Then the Tehran fire brigade turned up to put out – as one of the firemen later told me with infinite exhaustion – their 79th litter-bin fire of the night. I knew how he felt. A report that Basiji militia had taken over one of Mir-Hossein Mousavi's main election campaign office was a classic. Yes, there were uniformed men in the building – belonging to Mousavi's own hired security company.

Now for the very latest on the fantasy circuit. The cruel "Iranian" cops aren't Iranian at all. They are members of Lebanon's Hizbollah militia. I've had this one from two reporters, three phone callers (one from Lebanon) and a British politician. I've tried to talk to the cops. They cannot understand Arabic. They don't even look like Arabs, let alone Lebanese. The reality is that many of these street thugs have been brought in from Baluch areas and Zobal province, close to the Afghan border. Even more are Iranian Azeris. Their accents sound as strange to Tehranis as would a Belfast accent to a Cornishman hearing it for the first time.

Fantasy and reality make uneasy bedfellows, but once they are combined and spread with high-speed inaccuracy around the world, they are also lethal. Sham elections, the takeover of party offices, a massacre on a university campus, an imminent coup d'état, the possible overthrow of the whole 30-year old Islamic Republic, the isolation of an entire country as its communications are systematically shut down.

I am reminded of Eisenhower's comment to Foster Dulles when he sent him to London to close down Anthony Eden's crazed war in Suez. The secretary of state's job, Eisenhower instructed Dulles, was to say "Whoah, boy!" Good advice for those who believe in the Twitterers.

But the no-smoke-without-fire brigade has a point. Look at the extraordinary, million-strong march against the regime by Mousavi's supporters on Monday. Even the Iranian press was forced to report it, albeit on inside pages. Yes, the authorities have indeed closed down the local SMS service. Yes, they have slowed down – but not closed – the internet. My Beirut roaming phone now rarely reaches London, although incoming calls arrive – unfortunately for me – round the clock. The Iranian government is obviously trying to interfere with the communications of Mousavi supporters to prevent them from organising further marches. Outrageous in any normal country, perhaps. But this is not a normal country. It is a state as obsessed with the dangers of counter-revolution as the West is obsessed with Iran's nuclear ambitions. The Supreme Leader's speech yesterday was proof of that.

But then we had the famous instruction to journalists in Tehran from the Ministry of Islamic Guidance that they could no longer report opposition street demonstrations. I heard nothing of this. Indeed, the first clue came when I refused to be interviewed by CNN (because their coverage of the Middle East is so biased) and the woman calling me asked: "Why? Are you worried about your safety?" Fisk continued to spend 12 hours a day on the streets. I discovered there was a ban only when I read about it in The Independent. Maybe the Guidance lads and lassies couldn't get through on my mobile. But then, who had cut the phone lines?

We have, in fact, reported all the censorship – of local newspapers as well as communications. The footage of a brutal police force assaulting the political opposition on the streets of the capital has shocked the world. Rightly so, although no one has made comparison with police forces who batter demonstrators on the streets of Western Europe, who beat women with night-sticks, who have kicked over an innocent middle-aged man who immediately suffered a fatal heart attack, who have shot down an innocent passenger on the London Tube... There are special codes of morality to be applied to Middle East countries which definitely must not apply to us.

So let's take a look at those Iranian elections. A fraud, we believe. And I have the darkest doubts about those election figures which gave Mousavi a paltry 33.75 per cent of the vote. Indeed, I and a few Iranian friends calculated that if the government's polling-night statistics were correct, the Iranian election committee would have had to have counted five million votes in just two hours. But our coverage of this poll has been deeply flawed. Most visiting Western journalists stay in hotels in the wealthy, north Tehran suburbs, where tens of thousands of Mousavi supporters live, where it's easy to find educated translators who love Mousavi, where interviewees speak fluent English and readily denounce the spiritual and cultural and social stagnation of Iran's – let us speak frankly – semi-dictatorship.

But few news organisations have the facilities or the time or the money to travel around this 659,278 square-mile country – seven times the size of Britain – and interview even the tiniest fraction of its 71 million people. When I visited the slums of south Tehran on Friday, for example, I found that the number of Ahmadinejad supporters grew as Mousavi's support dribbled away. And I wondered whether, across the huge cities and vast deserts of Iran, a similar phenomenon might be discovered. A Channel 4 television crew, to its great credit, went down to Isfahan and the villages around that beautiful city and came back with a suspicion – unprovable, of course, anecdotal, but real – that Ahmadinejad just might have won the election.

This is also my suspicion: that Ahmadinejad might have scraped in, but not with the huge majority he was awarded. For with their usual, clumsy, autocratic behaviour, the clerics behind the Islamic Republic may have decreed that only a greater majority for the winner could decisively annihilate the reputation of its secular opponents. Perhaps Ahmadinejad got 51 per cent or 52 per cent and this was preposterously increased to 63 per cent. Perhaps Mousavi picked up 44 per cent or 45 per cent. I don't know. The Iranians will never know, even though the Supreme Leader told us yesterday that the incredible 63 per cent was credible. That is Iran's tragedy.

Yes, Ahmadinejad remains for me an outrageous president, one of those cracked political leaders – like Colonel Ghaddafi or Lebanon's General Michel Aoun – which this region sadly throws up, to the curses of its friends and to the delight of its enemies in the West. And the Islamic Republic itself – while it has understandable historical roots in the savagery of the Shah's regime which preceded it, not to mention the bravery of its people – is a dangerously contrived and inherently unfree state which was locked into immobility by an unworldly and now long-dead ayatollah.

And those nuclear arms? How many of us reported a blunt statement which the Supreme Leader and the man who ultimately controls all nuclear development in Iran made on 4 June, just eight days before the elections? "Nuclear weapons," he said in a speech in which he encouraged Iranians to vote, "are religiously forbidden (haram) in Islam and the Iranian people do not have such a weapon. But the Western countries and the US in particular, through false propaganda, claim that Iran seeks to build nuclear bombs – which is totally false..."

There are few provable assurances in the Middle East, often few facts and a lot of lies. Dangers are as thick as snakes in the desert. As I write, I have just received another call from Lebanon. "Mr Fisk, a girl has been shot in Iran. I have a video from the internet. You can see her body..." And you know what? I think he might be right.