Security officials in Egypt said an explosion had hit the bus on Thursday morning while it was travelling through Cairo's eastern Nasr City district.

Medical sources reported that five people had "mild to moderate injuries." A source speaking to the AP news agency on condition of anonymity said there was a suspicion that the device was thrown at the bus, or that it was exploded close to the vehicle.

The country's interior ministry said the bomb had been planted in a grassy area at the intersection of two busy streets. Another bomb was said to have been defused nearby.

Militants have killed scores of security personnel since the military's toppling of President Mohamed Morsi in July. However, Thursday's attack appeared to be the first bombing to have been carried out deliberately against a civilian target.

Hard line on Islamists

The government on Wednesday tightened its crackdown on Morsi's Muslim Brotherhood, formally listing the group as a terrorist organisation. The group was accused of carrying out a suicide bomb attack on a police station in the Nile delta city of Mansoura on Tuesday, in which 16 people died.

The moderately Islamist Brotherhood - which renounced violence in the 1970s - condemned that attack, which was in turn claimed by an Al-Qaeda-inspired group based in the Sinai Peninsula.

With the Brotherhood now listed as a terrorist organization, authorities have the power to charge any member of the group, along with anyone who finances the organization or promotes it in any way. The Brotherhood has said it remains committed to restoring Morsi, Egypt's first freely elected leader following the ouster of dictator President Hosni Mubarak in 2011, by peaceful means.

More than 2,000 Islamists have been arrested since Morsi was removed from power. Former Egyptian president Mohammed Morsi has been charged with conspiring with foreign organizations to commit acts of terrorism, prosecutors announced on December 18.

rc/ph (AFP, AP, dpa, Reuters)