Two explosions have been heard in the space of two minutes in a busy district near Egypt's capital.
Al Jazeera sources, quoting the security directorate, said the two blasts occurred in Giza and went off on Giza Bridge near where Central Security Forces vehicles were parked.
The AP news agency said six people were wounded in the attack including four Egyptian policemen, citing an interior ministry official.
There was no immediate claim of responsibility for the blasts.
Police cordoned off the scene, where a lightly damaged police truck appeared to have borne the brunt of the blast.
State television reported that the attack targeted a checkpoint set up to counter a scheduled protest by former president Mohamed Morsi's supporters, who had called for rallies later on Friday.
It is thought the explosive devices were homemade and detonated close to Istiqama Mosque and Giza Square.
The Health Ministry said it was too early for figures to be known.
Egypt has seen a wave of attacks claimed by armed groups since the army last year removed Morsi as president and cracked down on his supporters.
Sinai-based groups have previously claimed responsibility for attacks in Cairo and elsewhere.
An Al-Qaeda-inspired group based in the Sinai said it carried out a car bombing outside Cairo police headquarters on January 24 that killed four people.
Two more policemen were killed in three additional bombings in the capital that day.
Meanwhile, clashes between security forces and Islamist protesters left one person dead in the town of Fayoum, 80km southwest of Cairo.
The AP news agency quoted hospital officials saying one person was shot in the back and died immediately as protesters fought with security forces during a rally by supporters of the Muslim Brotherhood.
Source: Al Jazeera and agencies