The explosive was planted near the Ain Shams University and the Ministry of Defence in Abasya area here.
The device was discovered by a passersby, an official said.
Bomb-Disposal Squad rushed to the sites and four persons were injured while they were trying to defuse the bomb, the official added.
The injured includes an explosive expert.
Police cordoned-off the area and launched search operations. The university was also evacuated.
Militant attacks targeting police and military increased after the ouster of Islamist ex-president Mohamed Morsi last year. Since then over 500 security personnel have been reported killed.