Egyptian opposition protesters clashed with Islamists on Friday near the Muslim Brotherhood's headquarters after activists marched to the building guarded by police and Brotherhood members.
The two sides threw stones at each other near the building. Police who had been deployed outside the headquarters, have not yet intervened, an AFP correspondent said.
As the clashes continued, live television showed hundreds of protesters carrying anti-Brotherhood banners and making their way up to the hilly Mokattam neighbourhood where the Brotherhood headquarters are situated.
Opposition activists had called for the protest a week after they clashed with the Islamists near the headquarters. The Brotherhood on Thursday vowed it would protect its headquarters and bused in hundreds of supporters.
The Brotherhood has seen about 30 of its offices across the country attacked in widespread protests against President Mohamed Morsi, the Islamists' successful candidate in last June's election.
The Brotherhood, well-organised despite decades of persecution under former strongman Hosni Mubarak and his predecessors, was the main winner of parliamentary and senate elections last year.
But its critics accuse it and Morsi of mirroring the tactics used by Mubarak against the opposition.