EDITOR'S NOTE: Iranian authorities have barred journalists for international news organizations from reporting on the streets and ordered them to stay in their offices. This report is based on the accounts of witnesses reached in Iran and official statements carried on Iranian media.

Several thousand protesters — some chanting "Where is my vote?" — clashed with riot police in Tehran on Sunday as Iran detained local employees of the British Embassy, escalating the regime's standoff with the West and earning it a stinging rebuke from the European Union.

Witnesses said riot police used tear gas and clubs to break up a crowd of up to 3,000 protesters who had gathered near north Tehran's Ghoba Mosque in the country's first major post-election unrest in four days.

Some described scenes of brutality, telling The Associated Press that some protesters suffered broken bones and alleging that police beat an elderly woman, prompting a screaming match with young demonstrators who then fought back.

The reports could not be independently verified because of tight restrictions imposed on journalists in Iran.

North Tehran is a base of support for opposition leader Mir Hossein Mousavi, who has alleged massive fraud in Iran's disputed June 12 presidential election and insists he — not President Mahmoud Ahmadinejad — is the rightful winner.

Sunday's clashes erupted at a rally that had been planned to coincide with a memorial held each year for Ayatollah Mohammad Beheshti, who came to be considered a martyr in the Islamic Republic after he was killed in a major anti-regime bombing in 1981.

Witnesses said the protesters also chanted, "Ya Hussein, Mir Hossein," linking Mousavi's first name with a highly revered Shiite saint, Imam Hussein — the grandson of the Prophet Muhammad and a symbol of personal sacrifice for a cause. Later, after the situation calmed, police set up patrols and cordons.

It was Iran's first election-related unrest since Wednesday, when a small group of rock-throwing protesters who had gathered near parliament was quickly overwhelmed by police forces using tear gas and clubs.

Iranian authorities say 17 protesters and eight members of the volunteer Basij militia have been killed in two weeks of unrest, and that hundreds of people have been arrested.

The Paris-based International Federation of Human Rights said its information suggests at least 2,000 arrests have been made — "not just (people) arrested and later released, but who are locked up in prison," the group's vice president, Abdol Karim Lahidji, told the AP.