The state news agency MENA said 12 people were killed, including eight police officers, and that 134 were wounded, among them the city's security chief and his assistant.
Most of the victims were policemen, many of whom were buried beneath the debris.
Associated Press video from the scene showed bulldozers clearing the rubble outside the security headquarters, as charred and wrecked cars littered the street.
Egypt's Interior Minister Mohammed Ibrahim toured the scene of the explosion at daybreak, pledging that the police will “continue their battle against the dark terrorist forces that tried to tamper with the country's security,” then went to hospital to visit the wounded.
MENA quoted Cabinet spokesman Sherif Shawki as saying that the Brotherhood showed its “ugly face as a terrorist organization, shedding blood and messing with Egypt's security.”
Prime Minister Hazem el-Beblawi described the attack as a “terrorist incident,” expressed condolences to the families of the victims and vowed that the perpetrators “will not escape justice.”
A security official, speaking on condition of anonymity because he was not authorized to talk to the media, said the preliminary investigation indicated a car bomb caused the explosion, which also damaged a nearby bank and a theater.
