A car bomb struck the Egyptian police headquarters in downtown Cairo on Friday, killing at least three people and wounding dozens, the country's state media reported.
The blast, which could be heard in several parts of the city, sent smoke rising above the Egyptian capital as a large number of ambulances rushed to the scene.
State news agency MENA quoted an unnamed security official as saying the explosion was caused by a car bomb. At least three people were killed and 47 were wounded, according to the state radio.
The bombing came on the eve of the anniversary of the start of the 2011 uprising that toppled Egypt's longtime autocratic ruler Hosni Mubarak.
A Muslim Brotherhood-led coalition plans protests after Friday prayers across the country as part of their near-daily demonstrations against the July overthrow of Islamist President Mohammed Morsi and the recent vote on the country's rewritten constitution.
Shortly after the explosion, the state TV aired footage showing several wrecked and charred floors of the high-rise security building with the pavement outside littered with shuttered glass, pieces of bricks and rocks.
According to the report, gunfire resounded from the area immediately after the blast, but there were no other details.
About two hours later, rescue teams were still trying to extract victims trapped inside the security building, MENA said. The agency said the blast had also shattered windows and damaged the facades of a museum and a court house nearby.
No one immediately claimed responsibility for Friday's bombing but Egypt has seen a sharp rise in attacks targeting police and the military in the aftermath of the July 3 coup that ousted Morsi.
The most prominent attack was a failed assassination attempt on the interior minister in Cairo in September and the December suicide car bombing that targeted a security headquarters in the Nile Delta city of Mansoura, leaving nearly 16 dead, most of them policemen.
The military-backed government has blamed the Brotherhood group, from which Morsi hails, for the attacks, and designated it as a terrorist organization. The group has denied the accusations as baseless.
An al-Qaida-inspired group called Ansar Beit al-Maqdis, or the Champions of Jerusalem, has claimed responsibility for most of the recent attacks, saying they aimed to avenge the killings of Morsi's supporters in the months-long heavy security crackdown on protesters demanding his reinstatement and denouncing the coup.