At least six people have been killed in four bombings in the Egyptian capital, the latest in a series of attacks that have rattled the public and undermined confidence in the interim government.

The first and deadliest explosion tore through the security directorate in downtown Cairo, killing four people and injuring more than 70, according to the Health Ministry. Officers at the scene said it was a suicide car bombing.

The blast left a deep crater in the street, and was large enough to shatter windows in shops hundreds of meters away. It also caused extensive damage to the Islamic antiquities museum across the street.

Witnesses said they heard gunshots before the explosion. "It was around 7am this morning. I heard shots and then the ground shook, the windows shattered," said Mohamed Taher, sweeping broken glass outside his cafe near the bomb site.

CCTV footage aired on local television on Friday night showed a white pickup truck parked outside the security directorate for several minutes before the blast. One clip shows the pickup parking at 6:29am. The driver exits the vehicle seconds later, and climbs into a dark sedan which pulls up alongside.

Parking outside security buildings is forbidden, but another clip shows a person walking out of the security directorate, looking inside the pickup, then turning to walk slowly back into the building.

The Brotherhood issued a brief statement on Friday morning saying that it "strongly condemns the cowardly bombings in Cairo… and demands swift investigations".

About three hours after the first explosion, there was a second blast at a metro station in Dokki, on the west bank of the Nile. Police said it was caused by a bomb hidden in a bag. One security officer was killed, according to the Health Ministry, and at least eight others injured.

A third explosion outside a police station near the pyramids caused no casualties, and a fourth, near a cinema in Giza, killed one person and injured four. Mohamed Ibrahim, the interior minister, said the last bombing was aimed at the motorcade of the Giza security chief.

The bombings come one day before the third anniversary of Egypt's 2011 revolution, which removed the longtime president, Hosni Mubarak, from power. The interim government has asked supporters to come down to Tahrir Square and other.

Attacks have become increasingly routine in Cairo over the past few months. Last week, a small bomb exploded outside a courthouse in Imbaba, a neighborhood in western Cairo, and five people were injured last month in an attack on a bus in Nasr City.

PHOTO CAPTION

The damaged Cairo Security Directorate building is seen from the Islamic Art Museum, after a bomb attack in downtown Cairo, January 24, 2014.

Al-Jazeera