Protesters set flame to the ruling National Democratic Party headquarters in Downtown Cairo Friday around 6:30 PM.
President Hosni Mubarak is expected to address the nation tonight about the current situation.
Thousands flooded the area coming off of Qasr al-Nil Bridge after earlier driving security forces back into Tahrir Square.
The protesters broke through police cordons, entering the square in the evening and setting police vehicles on fire along the Corniche.
During the so-called "Day of Anger" protests, which raged for the fourth day Friday, demonstrators have targeted party buildings around the country.