Several thousand opponents of Egypt's Muslim Brotherhood clashed with supporters of the Islamist group near its headquarters in Cairo on Friday, and at least 40 people were wounded, authorities said.
Columns of riot police fired tear gas as the rival groups tussled in the streets around the Brotherhood headquarters. Protesters hurled Molotov cocktails and stones, a witness said.
Earlier in the day, Brotherhood supporters had arrived in the vicinity on buses, were showered with stones from the protesters and threw stones back.
About 40 people were wounded in the fighting, Health Ministry official Khaled al-Khatib told state news agency MENA. They included a former presidential candidate, liberal Khaled Ali, who was injured in the shoulder, MENA said.
State TV showed large plumes of black smoke rising from the surrounding streets which it said came from buses set on fire. The buses belonged to the Brotherhood, an official from the group told MENA, who said protesters had set them alight.
The Brotherhood, of which President Mohamed Mursi is a leading member, had 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.
In Egypt's second city Alexandria, a Brotherhood office was broken into and vandalised, said Anis al-Qadi, a spokesman for the Brotherhood in the city. Clashes also erupted in Mursi's home province of Sharqia, in the Nile Delta, state TV said.
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.