"It is still unclear what caused the explosion, but it seemsto be a big one that led to the collapse of parts of thesecurity building in Dakahlyia and we are expecting manyinjuries and potential deaths," the source said.
Another security source said the blast may have been causedby a car bomb, but it was not yet clear if it was suicide attackor not.
State TV said five people were killed and more than 100hurt. It said two top security officials were among the injured.
Egypt's Nile News TV cut into its late-night programming tourge people to go to hospitals to donate blood to the victims ofthe attack, which it described as the worst in the city'shistory.
Attacks on soldiers and policemen have sharply risen inEgypt since the Egyptian army ousted elected Islamist PresidentMohamed Morsi in July in the face of mass unrest against hisrule.
Most of the attacks have occurred in the Sinai, whichborders Israel and the Palestinian Gaza Strip. Around 200soldiers have died in Sinai alone since Morsi's ouster.