Riot police clashed with up to 3,000 protesters near a mosque in north Tehran on Sunday, using tear gas and truncheons to break up Iran’s first post-election demonstration in five days.

Witnesses said some protesters fought back, chanting: “Where is my vote?” Others described scenes of brutality — including the alleged police beating of an elderly woman — in the clashes around the Ghoba Mosque. The reports could not immediately be independently verified because of tight restrictions imposed on journalists in Iran.

North Tehran is a support base for opposition leader Mir Hussein Moussavi, 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 broke out 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 1981 anti-regime bombing.

Iran’s standoff with the West over its crackdown on opposition protesters escalated on Sunday after authorities detained several local employees of the British Embassy in Tehran — a move Britain’s foreign secretary called “harassment and intimidation”. The European Union condemned the arrests. Iranian media said eight local embassy staff were detained for an alleged role in post-election protests, but gave no further details. British Foreign Secretary David Miliband said “about nine” employees were detained and that four had been released.

EU foreign ministers meeting in Corfu, Greece, issued a statement condemning the arrests and calling for the release of all those still detained. The 27-nation bloc also denounced Iran’s continuing restrictions on journalists.