Tara Benyamin in Tehran: Tear gas was fired. Screams erupted. I felt helpless in the face of this state power

Large swathes of central Tehran resembled a war zone on Saturday. Black smoke billowed from fire barricades hastily lit to dispel tear-gas fumes, helicopters circled noisily in the sky, ranks of black-clad special forces hugged guns to block protesters from entering main streets, while masked militamen on motorbikes chased people down side streets with batons. Anguished screams and gunfire ripped through a city unaccustomed to such sounds.

"Iran is now Palestine!" came the taunting chant from stone-throwing protesters, turning on its head the Islamic Republic's demonisation of Israel. Others roared the taboo-shattering words "Death to Khamenei!"– the first ever public challenge voiced on the street against the man at the apex of the regime. Another chant was: "Coup government, step down, step down!" My friend Simin and I had taken a taxi as far as we could to the appointed route for Saturday's protest. Holding each other's hand tightly, our free arms raised in the "V" for victory sign, we walked towards Enqelab Square, only to find that protesters were hurrying back up the street, disheartened by a long line of anti-riot police who had cordoned off the road and were shouting at us to disperse. "Don't be afraid!" some in the crowd hissed, urging people to turn back and confront the batons.

Others were confusedly milling around, caught between fear and their desire to protest. Tear gas was fired. Panic ensued, screams erupted, and a stampede into side alleys began as the hated Basiji militia revved up their motorbikes and, spiked clubs swinging over their heads, began chasing people – including old women and young girls – and striking out at them.

I had never been in such a situation before. Since bombings during the Iran-Iraq war in the 1980s, Tehran has never witnessed any violence, and the raw terror mixed with adrenaline pounding through my body was alien to me. Simin pulled me into a doorway; it was a school that had opened its doors to shelter the protesters. Everyone was incredulous and incensed: "How can they do this to their compatriots?" It was hard to believe that Khamenei, who commands the armed forces, would resort to brute force over a disputed election when opposition marches last week had been conducted in utter peace and silence.

An older woman, in tears, begged the crowd to go home to avoid more bloodshed: "They have orders to open fire!" Indeed, we could hear gunfire at an unknown distance. But if anything, the brutal response had outraged people all the more.

We ventured out, and as Simin and I made our way from street to street, we saw the same scenario again and again: thousands fleeing from tear gas, taking shelter in houses, tending the bloodied; some chanting "Death to dictatorship", others bravely clashing with the armed forces (and reportedly even overpowering a few stray basij) but overwhelmingly, amazingly, no one – not even the women among the demonstrators – backing down. On the way back to northern Tehran, Chamran Expressway was jammed with cars and streaming with throngs of protesters walking back from the city centre. Drivers and passengers – perhaps not all, but a great many of them – waved "V" signs back at their fellow Mousavi supporters. The slogan being chanted uproariously, as smoke plumed only a few streets away and the choppers swooped overhead, was: "Until Ahmadinejad is gone, every day we'll carry on."

As yesterday's image of a 16-year-old girl lying in a pool of blood in Azadi Square aired on BBC Persian, IRIB, the state media, declared that people "causing bloodshed and chaos on the streets" were "terrorist groups led by foreign powers". I cried, feeling helpless in the face of such massive state power: guns, goons, and propaganda. But then I remembered that more than us, it is the Supreme Leader who is now jarred with a sense of deja-vu, as the people chant his name the way they did the Shah's 30 years ago.