Robert Tait in Tehran
At first the ear-splitting explosions, blazing bonfires and choking stench of teargas could have been mistaken for the prelude to a new Iranian revolution.
In reality the occasion, Chahar Shanbeh Souri, was celebratory. Iranians young and old, male and female, were gathered in Mirdamad Street, in one of Tehran's most affluent neighbourhoods, to mark the opening of festivities for Norouz, the new year in the ancient Zoroastrian calendar.
Nevertheless, subversion - along with noise and smoke - hung heavy in the air as fashionably dressed young men lobbed firecrackers and set flame to piles of rubbish at the kerbside. No one present seemed in any doubt about the political undercurrents.
"This is a way for people to use their national traditions to show their opposition to the regime," said a man called Reza, before hurrying away, saying that the security forces were lurking nearby.
Stretching back 2,500 years, the significance of Chahar Shanbeh Souri for Iranians is comparable to Christmas in the west. But its pre-Islamic roots have rendered it an object of hostility to the country's hardline clerical establishment. After the 1979 revolution, the authorities tried to ban the celebration, deeming it an affront to Islamic mores.
But with many Iranians chafing against the regime's austere brand of Shia Islam, the festival has increasingly been used to express displeasure with the government.
In previous years, pro-regime vigilantes have been deployed to break up such gatherings by force. This time, however, the regime has tacitly permitted the festivals.
But tolerance has its limits. As people gathered, squads of baton-wielding police officers sealed off the nearby Mohseni Square, lest it become a magnet for unmanageably large crowds. Several times they attacked, using their batons and firing teargas canisters.
Muhammad Ghodzi, 28, a student, bristled with anger as he criticised the regime. "We hate their brand of Islam because it spills blood," he said. "This is a sort of Islam that keeps people backward. But young people nowadays think."
Asked what kind of political system he wanted, he replied: "Democratic, with a separation between religion and politics ... We will sacrifice our lives for democracy and freedom."
But Nader's fast food restaurant nearby provided a poignant counterweight to the prevailing mood of rebellion. Young women sat, challenging the Islamic dress code with their gaudy headscarves pushed far back to emphasise an array of glamorous hairdos. A tannoy announcement exhorted them to "observe the regulations". They duly pulled their headscarves forward.