As mass street protests continue for a second day in the Iranian capital, Tehran, the country's top leader has lashed out at the United States, saying it is behind the mayhem.

Ayatollah Ali Khamenei, speaking over Iran's state-run television, accused Washington of trying to put a wedge between the Iranian people and their government. U.S. officials realize they cannot overthrow the Islamic regime by military means, he argued, and so it is trying to stir social unrest.

The broadcast of the ayatollah's speech followed just hours after parts of Tehran came to a standstill as thousands of demonstrators, mainly in cars, clogged the city's streets, horns blaring.

The biggest anti-government demonstrations in months appear to be gaining momentum. Demonstrators chanted, "Death to dictators", and for political freedom. Some young protesters shouted slogans against Ayatollah Khamenei, an act that in Iran is punishable by prison sentence.

Police and hard-line vigilantes kept demonstrators from approaching Tehran University buildings, the scene of violent unrest four years ago. In an apparent call for calm, Ayatollah Khamenei urged the Islamic militia to stay out of the protests.

Student organizers say they want a repeat of turbulent demonstrations in July 1999 that paralyzed the capital, to press their demands for reform.

The standoff between President Mohamed Khatami and the reform-led parliament and powerful hard-liners in the judiciary has widened.

Iran is also facing growing pressure from Washington. U.S. officials accuse Tehran of harboring members of the al-Qaida terrorist network and building nuclear weapons. They say they support what they see as the Iranian people's desire for political change.

Observers say U.S.-based opposition satellite-television channels are promoting the protests.