In previous years, Iran's Islamic government has sought to stop youths lighting bonfires and fireworks for the ancient Zoroastrian feast of Chaharshanbe Souri.

The festival took place on the last Tuesday night before the Iranian New Year on 21 March with specific areas in Tehran allocated to ensure the fireworks passed off more safely and, in many areas across the country, police simply stood by passively watching exuberant youths rather than clashing with them.

"(Last year) they closed the street and shot teargas into the crowd," said 18-year-old Omid, who on Tuesday night was mingling with a group of teenagers of both sexes.

'Pagan' dilemma

In Tehran's Taleghani park families gathered to watch the pyrotechnics.

"The police want to take part in the people's festival and prevent accidents at the same time," Mehdi Ahmadi, a spokesman for the police, said ahead of Tuesday's celebrations.

Iran has an awkward relationship with its Zoroastrian religion

The Islamic Republic has an awkward relationship with its ancient Zoroastrian religion, whose festivals are widely observed by Muslim Iranians.

Since the 1979 Islamic revolution, the government has usually tried to crack down on what it perceives as pagan partying, prompting clashes between police and youths testing the boundaries of Iran's social restrictions.

But analysts say since the election of conservative President Mahmoud Ahmadinejad last year, hardliners who opposed the liberal policies of former President Mohammad Khatami may have less need to flex their muscles.

People power

Others say the authorities do not want to alienate the people during Tehran's stand-off with the West over its nuclear ambitions.

"The situation dictates that the conservatives should not irritate the people while they face the nuclear dispute," said Ebrahim Yazdi, the leader of the Freedom Movement, a banned liberal party. "They seem to have understood that their pressurising methods are pointless in social fields."

"Tehran prosecutor's office does not oppose the happiness but ... it will confront those who intend to abuse the excitement and the traditions of the people," read a statement from the prosecutor general.

State media on Wednesday said five people had been killed and more than 300 injured in revelling across the country, significantly less than previous years

However a few revellers said that some of the fun had actually been taken out of the festival now that the authorities were more accepting.

"It is not much fun if there is no trouble doing it," said Avesta, 19, throwing a cracker into a crowd of shrieking teenage girls. "This is not so bad though. We mingle with the girls and shoot fireworks and crackers.”