Security forces have massively increased their presence in Iran's major cities in the face of new calls for protests around Nowruz, the Persian New Year.
Witnesses reported on Tuesday that units were stationed at many central locations in the capital Tehran.
Activists called for demonstrations to coincide with traditional celebrations leading up to the Persian New Year, which this year falls on March 21. Tuesday marked the festival of Chaharshanbe Suri, the day when friends and family traditionally gather and jump over fire pits.
Following the protest calls, senior police commanders threatened decisive action by security forces.
Videos showing the first clashes between young people and security forces were already circulating on social media in the early evening on Tuesday, with firecrackers and smoke bombs being set off.
A car exploded in the south of the capital Tehran. People also set fire to barricades on the streets in other parts of the country. According to Iranian media reports, at least seven people were killed in incidents involving fireworks.
Some six months after the latest wave of protests began in Iran, the political and spiritual leadership is under enormous pressure. The disturbances in the autumn plunged the Islamic Republic into one of the most serious crises in decades.
The trigger was the death of the Iranian Kurdish woman Jina "Mahsa" Amini in mid-September. She died in police custody after being arrested for violating Islamic dress codes.
