THE riot police learned a lesson in the early hours of the chilly Belgrade morning. Their job was to keep the students at bay, but the flak-jackets and the guns and riot shields were of little use against Aristotle, Socrates, Kant and Descartes.

On Valisa Street near Republic Square the young cops, many of them Bosnian Serbs, faced up to a challenge for which they were totally unprepared. It came from the students from the faculty of philosophy in Belgrade University whose tactic was to enlighten the policemen on the theories of the great philosophers.

Lectures were given, questions asked, marks awarded, and the eyelids of the tough young policemen began to sag. Boredom set in as the country cops were mercilessly exposed to the details of the various philosophical systems.

Some hours later, the riot squad's resistance had been worn down. Its members filed off into the warmth of their nearby buses, cigarettes drooping from their lips, the freezing air steaming as they muttered swear-words.

For the first time since Christmas the police cordon had been broken and the streets were open once more. Suddenly, grey Belgrade became as lively as Rio de Janeiro at Mardi Gras. There was dancing and singing and whistle blowing. The clanging of dustbin lids woke the older citizens. They decided to join in rather than complain about the exuberance of youth.

The ingenuity of the protesters had triumphed again, and by daybreak student spokesman Dusan Vasiljevic was claiming a famous victory. Serbia's President Slobodan Milosevic, who precipitated the crisis by cancelling the local election victories of the opposition Zajedno (Together) coalition, was on the run, he claimed.

"We are more motivated, more determined and more inventive than the police. We have proved this. The leadership has its back to the wall. It has lost the initiative, it has no coherent strategy."

But by lunchtime the riot police were back outside the Moskva Hotel on the main Terazije Street. Their opponents this time were older, less inventive but just as determined as the students, who were then sleeping off their euphoria. Zoran, a pensioner, marched smartly up to a captain of the riot police.

"Well," he said, "isn't this nice? You people are getting 1,600 deutschmarks a month for standing around blocking our way. In the meantime my pension is not being paid."

The officer listened politely but did not reply.

JUST down the road a long, white No 37 bus stopped in mid-route. Its driver claimed it had broken down, but in fact he was joining the demonstrations. Suddenly, cars "broke down", too, and the dreaded locally made Yugos were joined by a red Russian Zhiguli and representatives of the more prestigious Western marques.

Bonnets were lifted, serious faces peered at engine blocks, spectators grinned, the police unenthusiastically but grimly held their line, and the pressure increased on President Milosevic holed up in his mansion in the more salubrious Belgrade suburbs.

Serbia's political leader has shifted ground in the course of 52 days of demonstrations against him. Early on he, and the official media he controls, ignored the opposition. Then he tried counter-demonstrations by bussing in supporters from the provinces. In doing so he used a tactic which brought him to power in Belgrade in the first place. But the pro-Milosevic demonstrators were vastly outnumbered.

Intimidation came next. Young men in battle fatigues appeared on the streets at night and beat passers-by with baseball bats. One Zajedno supporter, Predrag Starcevic, died after a beating. But far from being intimidated, the opposition deemed him a hero and the demonstrations continued.

Then came the paltry concessions. Victory in the second city of Nis and in some minor regions was conceded to Zajedno, but this won no relief. An Organisation for Security and Co-Operation in Europe delegation, headed by former Spanish prime minister Felipe Gonzalez, had verified Zajedno victories in 15 of 18 towns and the opposition would not settle for less.

Out of the blue, amid a rigidly non-violent campaign, a bomb went off in the headquarters of the Yugoslav Left Union (JUL), headed by Mr Milosevic's wife, Mirjana Markovic.

Then the army, under Gen Momcilo Perisic, a ruthless attacker of Mostar in what is known here simply as "the war" told students it would not turn its tanks on the opposition.

Where can President Milosevic turn now? Some observers say we will have to wait until after Tuesday, the Orthodox New Year's Day, before moves are made. Then, they say, Mr Milosevic will clamp down and launch large-scale arrests of opposition demonstrators.

Others are not so sure. Milan and Goran, students on their way home for some sleep before rejoining the demos today, felt the police may not support the president if he demands too much off them.

"You should remember that these policemen are young guys from the Krajina [Serbian Croatia] and Bosnia. In the end they regard him as having sold them out."

Mira, another student, felt differently. "They will support him because they are being paid."

All three worried about trouble in the province of Kosovo, where ethnic Albanians are in the majority and complain of oppression. It was here that Mr Milosevic started his rise to political dominance by stirring up ethnic tensions.

His old tactic of bussing people to mass rallies having failed, he may, they said, resort to the other old stratagem of fomenting ethnic problems in an attempt to unite Serbs behind him against Albanians.

And last night tension rose in Kosovo when a former communist official and member of Milosevic's Socialist Party, Mr Maliq Shelloli, was shot dead in the southern city of Podujevo. An organisation calling itself the Kosovo Liberation Organisation has been distributing leaflets in the city threatening death to those who collaborate with the "Serbian occupiers".