WASHINGTON (AFP) - The United States hopes that violence doesn't spread in Iran, the White House said Wednesday, after riot police and opposition protestors clashed in central Tehran.

"We obviously have seen and are following the reports of this, and hope greatly that violence will not spread and, again, are watching that clearly and closely," said White House spokesman Robert Gibbs.

Riot police waded in with batons and tear gas to break up opposition protests in central Tehran on Wednesday, as huge crowds staged a noisy anti-US rally nearby to mark the storming of the American embassy by students 30 years ago.

Witnesses said the violent clashes occurred at Haft-e-Tir square in the heart of the capital where several hundred opposition supporters had gathered mid-morning to stage a demonstration against President Mahmoud Ahmadinejad.