Tens of thousands of people defied a huge police deployment Wednesday night by taking to an area around Tehran University in their cars to mark the fourth anniversary of bloody pro-democracy student riots, but authorities did at least manage to prevent renewed clashes.

Despite a blanket ban on any gatherings, masses of vehicles were seen jammed around Enghelab (revolution) Square and the tense city-centre campus well into the early hours of Thursday.

The university facility itself has closed by authorities in what had been part of a determined bid to prevent any display of public anger against the clerical regime on the increasingly symbolic date.

Hundreds of anti-riot police, decked out in combat fatigues and protective gear, lined the streets, while members of the hardline Basij and Ansar Hezbollah militias - fierce defenders of the nearly 25-year-old Islamic regime - whizzed around on motorcycles.

The bumper-to-bumper jam of around 10,000 cars - each carrying on average at least two people - was accompanied by a deafening cacophony of car horns as, in a repeat of anti-regime protests last month, residents made their frustrations heard.

But those who dared to sound their horns while passing rival gatherings of vigilante groups in the normally quiet nocturnal streets were quickly surrounded and harassed.

The bizarre logjam came despite concerted efforts by Iranian authorities to prevent any further expressions of anger over the country's seemingly intractable political deadlock between entrenched hardliners and elected reformist MPs loyal to embattled President Mohammad Khatami.

But in what looked like a well-planned operation, police did at least manage to prevent major clashes by keeping the traffic moving. Islamist vigilantes were also stopped from carrying their favoured sticks and chains, and were prevented by police from attacking their behind-the-wheel rivals.

Only one brief scuffle was reported, although several young people were seen being detained by police.

One Farsi-language news website, the usually well-informed Baztab, said 58 people were arrested in Tehran, although around the university hundreds of people were seen milling around on pavements with few problems.

The website also reported gatherings in the poor eastern Tehran Pars district and minor ones in the provinces. State media on Thursday made no reference to the gatherings.

Wednesday was the fourth anniversary of massive street clashes between pro-democracy students and police, during which at least one demonstrator was shot dead and hundreds of others arrested or injured.

Some of the students detained in the July 1999 unrest are still in jail.

The anniversary protest also comes hot on the heels of last month's 10 days of anti-regime demonstrations, marked by clashes and the chanting of virulent slogans targeting Iran's supreme leader Ayatollah Ali Khamenei - from whom hardliners take their cue - but also President Khatami.

The accompanying crackdown saw 4,000 people arrested, the hardline judiciary said.

Student activists have voiced increasing anger at Iran's clerical leaders in recent months, with patience also running out among strong Khatami supporters - many of whom complain the mild-mannered president has failed to carry through his pledge to bring "Islamic democracy" to the country.