Hundreds of Egyptian protesters and Muslim Brotherhood supporters hurled stones at each other close to the group's headquarters in Cairo on Friday, witnesses said.
Columns of riot police stood guard as protesters holding flags and banners packed the streets around the Brotherhood headquarters, footage on Al Jazeera and state TV showed. Dozens of people were wounded in the clashes, state television said.
Earlier in the day, Brotherhood supporters had arrived in the vicinity on buses and were showered with stones from the protesters, the witnesses said.
The Islamist group, of which President Mohamed Mursi is a leading member, vowed on Thursday to defend the building.
The Interior Ministry urged "revolutionary and political forces" to remain peaceful during the protests, saying in a statement it had sent riot police to protect property.
"The Interior Ministry has sent riot police forces to the headquarters of the Muslim Brotherhood to protect public and private property," it said in a statement.
Anti-Brotherhood protesters clashed with police outside the building this week, the latest burst of unrest in a nation still struggling to restore law and order since its 2011 uprising.
The police did not appear to have been involved in Friday's clashes between rival groups of demonstrators.
Although nationwide protests have dwindled since the end of last year when thousands took to the streets after Mursi gave himself sweeping powers, Egypt is still deeply split between Islamists, including the Brotherhood, and opposition groups.
Unrest has erupted in other Egyptian cities this month, including deadly clashes in Port Said, on the Suez Canal, between police and residents angered over death sentences handed down in a football riot court case.
The turmoil is hindering the efforts of Mursi, elected in June, to revive an economy in crisis and reverse a fall in Egypt's currency by luring back investors and tourists.
© 2014 Thomson/Reuters. All rights reserved.