It was one of those unpredictable Lebanese Sunday mornings. The ski slopes in the mountains overlooking Beirut would have been crowded with skiers enjoying the brilliant winter sunshine. Walkers were out along the Corniche, strolling in designer tracksuits. Downtown, the chic restaurants were preparing for lunchtime. And there were a few men on scooters riding around town broadcasting an imminent protest.

It wasn't long before the heavily-laden coaches and minivans began to arrive from Beirut and the rest of Lebanon. They were all full of young, often bearded men who wore headbands and carried identical flags with calligraphic inscriptions in Arabic such as: "There is no god but God and Mohammad is his Prophet" and "O Nation of Muhammad, Wake Up."

There were soon as many as 20,000 of them filling the streets. They walked up past the Christian quarter of Gemmayze and into the even more genteel Christian area of Achrafieh, gathering not far from the Danish embassy, the target of their protest. One man waved a placard in English that said: "Damn your beliefs and your liberty." Another carried a sign saying: "Whoever insults Prophet Muhammad is to be killed."

The police seemed to know the demonstrators were coming and had turned out in force with barriers, barbed wire fences and several large fire trucks. Just a day earlier, the Danish and Norwegian embassies in Damascus had been torched by a furious mob, repeating the violent protests that have spread across the world from Gaza to Afghanistan to London. On Saturday night, anticipating trouble, the Danish diplomatic staff in Beirut flew home.

The mob stood in the street, chanting their fierce condemnation of the Danish cartoons that spawned this rapidly-spreading crisis. By 11am, the Lebanese police and army were firing tear gas at the crowd. The protesters threw volleys of stones. Some stuffed cotton wool into their nostrils to stifle the effect of the gas.

One group overturned a car and set it alight. Sunni clerics in robes tried to calm the young men down. They were ignored. One cleric, Ibrahim Ibrahim, said his pleas were met with stones and insults. "They are hooligans," he said.

The mob grew fiercer, and finally the police withdrew. As they moved back, the crowd smashed their way into the building housing the Danish embassy and set it ablaze. From the burning building they hung a banner that read: "We are ready to sacrifice our children for you, O Prophet Muhammad." By now dozens of people had been wounded or arrested and at least one person was killed, a protester apparently caught up in the fire at the embassy building.

The many politicians representing Lebanon's fractured sectarian society sensed this was suddenly a situation a long way out of control. "It is the work of infiltrators," said Saiad Hariri, a prominent Sunni politician. "These acts have nothing to do with the Prophet. They are harming Muslims."

On the street, the riot began to take a more sectarian turn. Throwing the metal barriers and barbed wire aside they chased the police up into the narrow alleys of Achrafieh, well beyond the embassy and deep into the Christian quarter. They smashed dozens of parked cars and tossed bricks through the windows of the furniture boutiques and hair salons. Others overturned two police cars and threw rocks through the windows of the St Maron church.

"What is the guilt of the citizens of Achrafieh for caricatures published in Denmark?" said Charles Rizk, the justice minister and a Christian. "This sabotage should stop."

Asad Harmoush, a leader of Jamaía Islamiya, the conservative Sunni Muslim group that had helped organise the protest, tried to deflect the blame. "We can't control tens of thousands of people. We tried to limit the harm and we extend our excuses to our brothers in Achrafieh and to the security forces. There has to be an investigation. Obviously there were infiltrators."

And then in the early afternoon, as suddenly as it had all begun, it ended. The leaders of the mob turned to the angry young men beside them and told them it was time to leave. Obediently the crowd thinned out and began walking back to the buses, even as the Danish embassy continued to burn. By 3pm there wasn't a single protester left on the street. Later, the Lebanese interior minister, Hassan Sabei, announced his resignation.

The police returned in force, and with nothing to do they began taking photographs of each other in front of the burned-out building. Firemen hosed down the blaze. Crowds of Filipino maids returned from their day off back to their jobs in the homes of the wealthy, while the wealthy were out patching up their cars. Dozens of street sweepers hosed down the roads and collected the debris of the day.