Egyptian state television is reporting a large explosion near police headquarters in Cairo, just a day before the sensitive third anniversary of the country's uprising.
There was no immediate word on casualties from the blast. It occurred in the mostly deserted downtown area of the Egyptian capital in the early morning hours of Friday, the first day of the Egyptian weekend.
The explosion was heard in many areas throughout the capital, awaking startled residents, many of whom posted pictures on Twitter showing thick black smoke rising from the city skyline.
Other pictures from the scene showed extensive damage to the police headquarters, with windows and walls blown out and a large crater in the street. The explosion also damaged the nearby Islamic Museum.
Authorities have not said what caused the blast.
Islamist militants have been blamed for a number of attacks against Egyptian security forces since the country's military ousted President Mohamed Morsi in July.
Mr. Morsi, Egypt's first democratically elected president, came to power following the 2011 ouster of longtime President Hosni Mubarak.
The Brotherhood has denied government accusations that it took part in any of the attacks. But it has continued to hold regular anti-coup protests, some of which have turned violent.
The Brotherhood and Egypt's interim government have called for rival protests on Saturday, the third anniversary of the country's mass protest movement.