(Adds details, background)
CAIRO, Dec 26 (Reuters) - A bomb blast hit a bus in Cairo's Nasr City district on Thursday, injuring four people, a spokesman for Egypt's interior ministry told Reuters, two days after a car bomb killed 16 in the Nile Delta.
The homemade device had been put on a traffic island and a second one, found nearby, was being dismantled, Hani Abdel Latif said. State television showed footage of a bus with its windows blown out.
Attacks across Egypt over the last few days have raised fears of intensifying violence in the run-up to a constitutional referendum in January as the interim government pushes ahead with an army-backed plan for political transition.
The government has vowed to fight "black terrorism" after Tuesday's deadly attack on a police compound in the city of Mansoura which also injured about 140 people.
Bomb attacks and shootings have become an almost daily occurrence in the Sinai peninsula since the army toppled Islamist President Mohamed Mursi in July following mass protests against his rule. (Reporting by Shadia Nasralla; Editing by Louise Ireland)