A bomb blast hit a Cairo bus on Thursday, injuring at least four people.
Egypt's interior ministry says the homemade device was placed on a traffic island.
A second device found nearby was dismantled.
The blast is the latest in a series of attacks across Egypt over the last few days.
A car bomb killed 16 in Egypt's Nile Delta on Wednesday.
And a deadly attack on a police compound in the city of Mansoura the day before injured some 140 people.
Bomb attacks and shootings have become an almost daily occurrence in Egypt's Sinai peninsula since July.
That's when the army toppled Islamist President Mohamed Mursi following mass protests against his rule.
The recent attacks raise fears of intensifying violence in the run-up to a constitutional referendum in January.
