The headquarters of the moderate Egyptian Islamist party Wasat was set on fire on Monday, in an expansion of attacks on Islamist organisations across the country.
Unidentified assailants threw petrol bombs at the building.
Earlier on Monday, the headquarters of the ruling Muslim Brotherhood was overrun by youths who ransacked the building after those inside were evacuated following a night of violence that killed eight people.
The Wasat Party was founded in the 1990s by former members of the ruling Muslim Brotherhood but was not officially permitted until after the 2011 uprising that overthrew Hosni Mubarak.
The party has allied itself with the Brotherhood since President Mohamed Mursi took office a year ago. It proposed a draft law for judicial reform in the Islamist-dominated upper house of parliament, sparking a revolt among thousands of judges earlier this year.