Iranians have celebrated an ancient festival despite the deployment of hundreds of extra police across Tehran to discourage anti-government protests, BBC reported.
Officials had warned the opposition against staging demonstrations on Chaharshanbeh-Suri, held a few days before Norouz, the Persian New Year.
Some opposition websites reported sporadic clashes in the capital, while police said 50 people were arrested.
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.
The eve of the festival is marked by special customs and rituals, most notably jumping over bonfires and setting of fireworks.
The authorities have dismissed the celebrations as heretical fire-worship.
Iran's Supreme Leader, Ayatollah Ali Khamenei, said the festival 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 hardliners 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.