Iranian police used tear gas and electric prods to crack down on the country's biggest antigovernment protests in at least a year, as demonstrators buoyed by activism across the Middle East returned to the country's streets by the tens of thousands Monday.

The day of planned antigovernment rallies began largely peacefully, according to witnesses, with protesters marching silently or sitting and chanting. But as demonstrators' ranks swelled, police and antiriot forces lined the streets, ordered shops to shut down and responded at times with force, according to witnesses and opposition websites, in a repeat of the official crackdown that helped snuff out months of spirited opposition rallies a year ago.

By day's end, online videos showed garbage bins on fire, protesters throwing rocks at the police and crowds clashing with motorcycle-mounted members of the pro-regime Basij militia.

Monday's protests come as calls for regime change have led to the popular ousters of Egypt's Hosni Mubarak and Tunisia's Zine al-Abidine Ben Ali. They mark a broadening from Iranian rallies that drew hundreds of thousands through 2009 and early 2010.

Those rallies targeted what opposition leaders said was a flawed presidential election that they say unfairly returned President Mahmoud Ahmadinejad to power. Monday's protests, by comparison, demanded that Supreme Leader Ayatollah Ali Khamenei, the core of power in the Islamic Republic, step down.