The Damietta offices of the formerly ruling National Democratic Party (NDP) caught fire on Sunday.
Fire reached the building's top floor, where the NDP used to hold its meetings.
Firefighters responded with 12 fire engines and managed to put the fire under control, but with difficulty.
Initial findings point to criminal motivations behind the blaze, said security sources who asked not to be named. They said the building has been closed for months after court rulings disbanded the NDP.
In April, the Supreme Administrative Court dissolved the NDP, which ruled throughout the 30-year presidency of Hosni Mubarak. It also ordered that the party's offices be returned to the state.
The NDP, established by former President Anwar Sadat, had two offices in each governorate.
Most were palaces that were nationalized following the 1952 Revolution. They were later turned into offices for the Arab Socialist Union, the only political organization affiliated to the regime during the 1960s and 1970s. The NDP took control of the buildings after its establishment.
Translated from the Arabic Edition