Last Updated Jan 24, 2014 4:09 PM EST
CAIRO -- A wave of bomb attacks hit Cairo on Friday, killing six people and raising fears that an Islamist insurgency is gaining pace on the eve of the third anniversary of the uprising that toppled autocrat Hosni Mubarak.
The violence underscored the struggle of authorities to tame an Islamist insurgency which has been gaining pace since the army toppled Islamist President Mohamed Mursi in July.
In the most high-profile attack, a car bomb exploded at a security compound in central Cairo early in the morning and killed at least four people, including three policemen, security sources said.
They said the blast was the work of a suicide bomber. But footage broadcast on an Egyptian television channel showed a man getting out of a van and moving into another vehicle. Minutes later the van exploded.
There was no immediate claim of responsibility for the attack in the parking lot of the Cairo Security Directorate, or the other blasts. But they had all the trademarks of attacks carried out by militants seeking to topple the army-backed government.
Another blast in the Dokki district killed one person. An explosion near a cinema on the road to the Pyramids of Giza on the outskirts of Cairo also led to one fatality.
In Washington, the White House condemned the bombings and urged all sides to avoid violence. "These crimes should be investigated fully and the perpetrators should be brought to justice," spokesman Jay Carney told reporters.
Clashes in the capital and several other cities between Mursi supporters and security forces which killed 11 people also raised tensions in the biggest Arab nation.
An eyewitness to the second explosion in the Cairo neighborhood of Dokki told CBS News' Alex Ortiz a device was placed under a police truck.
At the police headquarters, an Associated Press photographer said about six police officers sat on the sidewalk outside the building and wept. Small parts of a vehicle could be seen scattered around on the road and a corpse -- which officers said was a suicide bomber -- lay in a pool of blood under a blanket near a scorched car engine.
Several floors of the high-rise security building were wrecked, air conditioning units dangled from broken windows, and the pavement outside was covered with piles of shattered glass, pieces of bricks and rocks. The facade of the adjacent Islamic Art Museum and a court house were also damaged, along with shops and cars in the area.
The Interior Ministry cordoned off the building, which is located in a busy district, as rescue teams worked to extract victims trapped in the rubble. Security forces went on high alert, and closed the central Tahrir Square and main roads, including the one leading to the Interior Ministry.
Prime Minister Hazem el-Beblawi condemned the bloodshed in a statement, saying it was an attempt by "terrorist forces" to derail the army-backed government's political road map, which is meant to lead to free and fair elections.
Interior Ministry spokesman Spokesman Gen.
Hani Abdel-Latif said the blast was caused by a car bomb that struck cement
blocks placed five yards from the outside gates of the building. He declined to
comment on earlier reports of subsequent gunfire or that the guards opened fire
on a suicide car bomber.
State television quoted witnesses as saying gunmen on motorcycles opened fire on buildings after the explosion.
Egypt plunged in bouts of violence after the July 3 military coup against Morsi and a heavy-handed security crackdown on the Muslim Brotherhood that left hundreds dead and thousands arrested.
A Muslim Brotherhood-led coalition had planned
protests after Friday prayers across the country as part of their near-daily
demonstrations against Morsi's overthrow and the recent vote on the country's rewritten constitution.
Car bombings are common in some areas of
Egypt, including the Sinai Peninsula, but they are rare in the capital.
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 Muslim Brotherhood for the attacks and designated it as a terrorist organization. The group has denied the accusations as baseless.
An al-Qaeda-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.