At least 12 people were killed and more than 100 wounded in a suspected car bomb attack at a security building north of Cairo on Tuesday.
The blast ripped through a police headquarters in the city of Mansoura at around one in the morning, leaving scores of people buried under the rubble.
Security forces cordoned off the area around the site, closed major entrances and exits to the city, and set up checkpoints to search for perpetrators.
State television called on residents to donate blood at hospitals.
A security official said preliminary investigations showed a car bomb caused the explosion, which also damaged a nearby bank and a theatre.
The incident was the first major bombing in the Nile Delta, pushing the country’s unrest nearer to Cairo.
No groups immediately claimed responsibility for the bombing.
However, a militant group based in Sinai and several others have claimed responsibility for a surge of attacks on security forces since the country's former president, Mohammed Morsi, was toppled in July.
The country’s interim government blamed the pro-Morsi Muslim Brotherhood for the attack.
Prime Minister Hazem el-Beblawi described the attack as a "terrorist incident," and expressed condolences to the families of the victims and vowed that the perpetrators "will not escape justice."
Tuesday's bombing was not the first incident where the security headquarters in Mansoura was targeted. Weeks ago, an explosion went off in front of the building but caused no casualties.
Since Morsi was ousted, opposition forces have attacked several security headquarters in the country with car bombs or by suicide bombers.
Additional reporting by Associated Press