The fire festival of ChaharShanbeh- Suri predates Islam
Iranian police say they have arrested 50 people during clashes between opposition supporters and police in Tehran during a new year festival.
The Feast of Fire comes on the eve of the Persian new year, but religious leaders had told Iranians celebrating it was "un-Islamic".
There were a number of clashes between young people and the police across the capital, a Tehran police chief said.
Opposition leaders had told people not to protest during the festival.
The BBC's correspondent Jon Leyne says there is no sign these clashes will widen into a bigger political protest.
The clashes were more a show of defiance against demands from religious authorities that Iranians refuse to celebrate the festival, our correspondent says.
Young Iranians went out into the street to leap though bonfires and set off firecrackers, traditional on Chaharshanbeh-Suri - or Red Wednesday - held a few days before Norouz, the Persian new year.
Large protests have been held during public events since June's disputed election.
Opposition groups say it was rigged to ensure the re-election of President Mahmoud Ahmadinejad, a charge the government denies.
At least 30 protesters have been killed in clashes since the poll, although the opposition says more than 70 have died. Thousands have been detained and some 200 activists remain behind bars.
In December, eight people were killed in clashes on Ashura, one of the holiest days in the Shia Muslim calendar.
Held on the last Wednesday before Norouz, Chaharshanbeh-Suri celebrates the coming of spring and dates back to Zoroastrian Persia, which existed centuries before the coming of Islam.
Iran's Supreme Leader, Ayatollah Ali Khamenei, said the festival was heretical fire worship and had no basis in Sharia law.
"It creates a lot of harm and corruption which is why it is appropriate to avoid it," he told reporters.
Before the festival, opposition leaders had called for supporters not to use the festivities to protest, and that they should not provoke the security forces.
Nevertheless, the opposition Jaras website said there had been clashes in several parts of Tehran. The reports could not be verified.
Speaking to the Isna news agency late on Tuesday, Tehran's deputy police chief Ahmad Reza Radan said only that 50 people had been arrested for causing "an unacceptable level of nuisance to the public".
Opposition supporters have faced increasing pressure from the authorities, with some hard-liners labelling them as "mohareb" - enemies of God who can be sentenced to death under Sharia law.
At least nine have been sentenced to death and two reportedly hanged.