A third blast struck near police deployed outside Cairo University close to where an earlier twin bombing killed a police general on Wednesday Al Arabiya reported.
The blast struck near the gates of the university in the center of the Egyptian capital and close to a roundabout where the two bombs exploded close to police posts.
There was no immediate word on casualties in the latest blast.
The explosion appeared to be much smaller than two bombings that struck at the same location, less than two hours earlier.
Those explosions killed at least two people - a police brigadier general and a civilian - and wounded seven, three senior police officers and four civilians.
The slain officer was identified as Brigadier General Tarek al-Mergawi.
Among the wounded was Maj. Gen. Abdel-Raouf El-Sirafy who is the deputy chief of police in the Giza province, parts of which are located in the Greater Cairo area.
Cairo's University main campus is in Giza.
Officials say the bombs went off seconds apart, targeting the police deployed in the area in anticipation of students loyal to ousted Islamist President Mohammad Mursi.
Egyptian state TV says the bombs were crude and homemade.
The officials spoke on condition of anonymity because they were not authorized to speak to the media.
Wednesday's attack takes to a new level the violence which has come to define protests by students in the nearly nine months since the military ousted Mursi. While the attacks have usually taken place in the restive Sinai Peninsula, they have recently spread to Cairo.
The government announced this week that nearly 500 people, most of them soldiers and police, have died from such attacks.
A video obtained by privately-owned Egyptian daily Youm 7 shows the first moments of the two blasts that took off outside the university.
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