DHAKA: A plume of black smoke rose over Cairo early Friday after a powerful explosion hit the city`s police headquarters, killing at least four people and wounding more than 50 others, Egyptian authorities said.

The blast struck a key symbol of authority in a country that has been shaken in recent years by political turmoil and violent unrest, reports CNN.

It was followed by another attack that wounded several people in a different Cairo neighborhood.

The explosions took place at time of high tension, the day before the third anniversary of the 2011 revolution that eventually brought down the country`s former authoritarian leader Hosni Mubarak.

The first blast appeared to have been caused by a suicide attacker who tried to drive a vehicle laden with explosives into the police headquarters, said Maj. Gen. Hany Abdel Latif, a spokesman for the Interior Ministry, citing preliminary reports.

Guards stationed in front of the headquarters in the Abdeen district of Cairo opened fire at the vehicle, and the explosion went off in the building`s vicinity rather than inside, he told state news agency Egynews.

The force of the blast destroyed the front of the first and second floors of the eight-story building, he said, and caused material damage on the third floor. Most of the building`s windows appeared to have been blown out.

At least 51 people were wounded in the explosion, state-run broadcaster Masriya TV reported, citing the Health Ministry.

CNN`s Reza Sayah said the blast appeared to be "the most powerful bomb attack that we`ve seen here in central Cairo in recent memory."

Speaking from near the scene of the blast, Sayah said the attack will likely intensify the fight between Egypt`s military-backed government and the groups that oppose it.

It wasn`t immediately clear who was behind that attack.

Friday is a holiday in Egypt, so the police headquarters is unlikely to have been as busy as it would have been on a weekday. The blast happened around 6:30 a.m., according to state media.

Footage aired by local broadcasters showed significant damage to the front of the building. The street in front of the headquarters was cordoned off.

Hundreds of people -- some stunned, some angry -- gathered around the scene, Sayah said.

The explosion appeared to have shattered windows and damaged other parts of buildings several blocks away, he said.

A second explosion Friday went off near police station in Dokki, a residential area of Cairo, wounding several people, authorities said.

BDST: 1447 HRS, JAN 24, 2014