Six policemen have been killed in a blast on a main street in Cairo, an Egyptian security source says. Egypt's state-run news agency says an explosion in Cairo has killed six police and wounded another three. MENA says the explosion took place early today on Pyramids road, the main avenue leading from the city center out to the Giza pyramids. A crude explosive device went off near a checkpoint in Haram street, close to the Egyptian capital's famous Pyramids. Three people were injured in the explosion, the source added on condition of anonymity, without giving further details about their identities. Police forces have cordoned off the area and explosives experts are currently combing it for other possible devices. No group has yet claimed responsibility for the attack. Egyptian security forces have been targeted by frequent attacks since the removal of Islamist president Mohammed Morsi in 2013 by the army following mass protests that demanded his ouster. On Thursday, Egyptian authorities arrested Morsi's son on charges of inciting violence.