The motorcycle police came from behind. They fired stun grenades that exploded as I was walking among thousands of demonstrators on Tehran’s central boulevard, talking to two young women about their anger at what they called the “theft” of the Iranian election.

Tempers ran high. Protesters jostling shoulder to shoulder filled the road and pavements, punching the air with their fists and shouting, “Down with the dictator,” and, “Be ashamed and give us back Iran.”

They were convinced their candidate in the presidential election, Mir Hossein Mousavi, a former prime minister, had been cheated of victory by the government of President Mahmoud Ahmadinejad.

The election had been hard fought — Mousavi and Ahmadinejad were the frontrunners in a field of four — and an evidently tight race had been predicted to go to a second round this Friday.

It was an emotionally charged campaign. Mousavi’s supporters, many of them young men and women, nearly shut down the city last week with all-night street campaigning, dancing until dawn as they handed out campaign leaflets.

Such a spectacle had never been seen in a conservative city where women are legally obliged to wear the hijab (headscarf) and an overcoat, and the sexes are not allowed to mix unless married or related.

Rallies for both candidates drew tens of thousands of fervent supporters.

Just as they had all week, yesterday’s demonstrators waved green ribbons and banners and wore green T-shirts signalling their support for Mousavi.

“The election was stolen,” Safoor Nayafi, 26, shouted over the din of the march, clutching her black hijab at her chin. “We are marching to the ministry of interior to get our stolen votes back.” She was just telling me she had a master’s degree in science when her voice was drowned out by the roar of motorcycles from behind us and along the sides of the road.

They were riot police, dressed in camouflage uniforms, wearing black flak jackets and black helmets with their menacing visors pulled down.

They fired several grenades, scattering the crowd. Women screamed and fell to the ground; men leapt onto the pavements, then ran back to drag the fallen out of the road. Shop owners pulled scared bystanders inside and slammed down their metal grates.

It took only seconds to realise they were stun grenades, fired into the air to scatter the marchers, but they were terrifying seconds.

An acrid smell hung in the air as young men started shouting and running forward after the riot police. They were from a special unit trained to put down protests and named after Hassan Nasrallah, the Lebanese Hezbollah leader. Confronting them seemed a brave, but foolish, gesture.

About 50 yards ahead of us, at the junction of Vali-asr Street and a square called Saei Park, there was a skirmish, with motorcycles roaring and fired-up young men yelling, “Allahu akbar” (God is great).

Smoke rose from the scene as the crowd cleared. Two riot police motorcycles lay burning in the middle of the junction, flames leaping from their metal carcasses and smoke rising into the overcast sky. A loud cheer went up from the crowd.

The demonstrators were not seasoned fighters. Tehran residents told me they had never seen such a crowd — young men and women, intellectuals, workers.

One elderly woman came up to me, screaming: “We hate this government. It’s my generation’s fault to have let them come in 1979 (when the shah was ousted). These children are doing what we were not brave enough to do.”

The riot police returned with much roaring of engines and more wielding of stun grenades, but the marchers stubbornly regrouped and the stand-off continued.

Walking back to my hotel, I saw burning dustbins surrounded by riot police swinging their batons at bystanders — they had clearly been set alight by protesters. A colleague said he had counted four more burnt-out police motorcycles and seen dozens of riot police beating people in the street.

Clashes continued into the night and there were reports of one person being killed.

No one knew what to expect next. I asked one of the march’s leaders, sweating in his black T-shirt and green wristband, if the protests would turn into a fully fledged revolution.

“I don’t know,” he said. “This is new to us.”

The first protest had begun at about 4am, as Mousavi supporters, who had stayed up all night to wait for the results, converged on the ministry of interior, the department in charge of the poll, in the centre of the city. They were beaten back by riot police, who seemed to have been expecting them.

Later in the morning, I visited Mousavi’s headquarters in a five-storey building on a residential street. The riot police had already been there.

A guard in the lobby of the building told me they had stormed in about three hours earlier, firing tear-gas canisters and shouting that they had seen a suspect flee into the building. They beat people with batons, the guard said, and Mousavi had ordered the headquarters to be closed to stop any more violence.

The disappointment at Mousavi’s defeat quickly turned to anger as people absorbed the scale of Ahmadinejad’s victory.

Polls in Iran are unreliable, but every unofficial sampling had indicated a close race. The outpouring of support for Mousavi in Tehran could have been deceptive as Ahmadinejad’s supporters are mostly the rural poor and urban working class, but the gap between the two candidates caused Iranian analysts and voters alike to suspect that something was amiss.

Mousavi was ensconced with advisers last night, but said he would challenge the result: “I will not surrender to this dangerous charade.”

He called on his supporters to remain calm until he decided his course of action. Few seemed to be listening, however. Another demonstration was planned in central Tehran for late last night.

Ayatollah Ali Khamenei, Iran’s supreme leader, said the election result should stand. “Other honourable candidates must refrain from any kind of provocative and distrustful words or deeds,” he said on state radio.

News of the clashes travelled only by word of mouth; state-run television was still showing film of queues of voters on Friday. The texting network that Mousavi’s supporters had used to organise their campaign was blocked.

Even before yesterday’s riots, it was clear many ordinary people felt they had been cheated. “Almost everybody I know voted for Mousavi but Ahmadinejad is being declared the winner. The government announcement is nothing but widespread fraud. It is very, very disappointing. I’ll never again vote in Iran,” said Nasser Amiri, a Tehran hospital clerk.

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