Anti-government protesters have stormed a Muslim Brotherhood office in the Egyptian capital, Cairo, Press TV reported.
The Muslim Brotherhood (MB) says a group of men assaulted people inside one of its offices and ransacked the building on Friday.
Reports say anti-government protesters and MB supporters also clashed outside the group's headquarters in al-Muqattam district near central Cairo.
The protest against President Mohamed Morsi has been called by over 50 political parties and figures who have also demanded snap presidential polls and a new constitution.
The protesters accuse Morsi of using his power to promote the interests of the Muslim Brotherhood.
On Thursday, the Muslim Brotherhood vowed to defend its buildings against demonstrators.
"We have the right to defend our buildings, using all possible ways, if the police force does not defend them," said Brotherhood Secretary General Mahmud Hussein at a press conference.
Egyptians launched a revolution against Hosni Mubarak's regime in January 2011, which brought an end to over three decades of dictatorship by him in February 2011.