The bombing of a security building in Greater Cairo in the early hours of Thursday was caused by a car "which suddenly stopped" outside the building, a security source said.
The driver fled the scene on a motorcycle which had been following the car, the security source said in a statement issued by the Interior Ministry.
The explosion injured six policemen, who have been hospitalised, the source said. Earlier, state-television reported that eight were injured in the bombing.
The blast took place in the vicinity of the the Homeland Security building in northern tip of Cairo in Qalyubia governorate. Homeland Security, previously known as State Security, is a police branch that was renamed after the January 2011 uprising.
The blast has shattered windows, the facade of the building and part of the outer wall, the ministry's statement added.
The explosion, which sent shock waves across Greater Cairo, has also damaged nearby houses.
Many in central Cairo immediately took to Twitter, to report hearing the sound, some even reporting shattered glass.
People on social media have reported hearing the explosion in the neighbourhoods of Dokki, Haram, Zamalek, Maadi, cental Cairo and even in outskirts of the city.
Although militancy in Egypt has largely affected North Sinai, the Egyptian capital has been hit by several blasts.
The Italian consulate in Cairo was targeted on July 11, in an explosion which left one person dead and prompted the consulate to temporarily suspend its services.
Late Prosecutor General Hisham Barakat, who served as the state's highest prosecutor, was assassinated on June 29 when a bomb targeted his motorcade in Cairo.
