Chahārshanbe-Sūri is an ancient festival from the Zoroastrian tradition that is celebrated the Wednesday before Norouz, the Iranian New Year that falls on 21 March.
The night before, celebrants set fireworks, make seven bonfires in the streets and jump over them to mark the passage from winter to spring.
The authorities have dismissed the celebrations as heretical fire-worship without any basis in Sharia.
A few days ago, Supreme Leader Alì Khamenei said that it “creates a lot of harm and corruption, which is why it is appropriate to avoid it,"
The authorities actually fear that the opposition will use the festival and the public gatherings it generates to demonstrate as it did on other occasions following Mahmoud Ahmadinejad’s re-election and the emergence of the green wave.
Some opposition groups have called for demonstrations against the regime, but one leader, Mir Hossein Moussavi, has called on his supporters not to cause any turmoil.
Opposition website Jaras, reported clashes in several parts of Tehran. Other witnesses said that celebrations are taking place with greater discretion but without a glitch.
Nevertheless, police announced that it arrested 50 people for causing "an unacceptable level of nuisance to the public".