CAIRO (AP) -- A string of bombings hit police around Cairo on Friday, including a suicide car blast that ripped through the city's main police headquarters and wrecked a nearby museum of Islamic artifacts. Five people were killed in the most significant attack yet in the Egyptian capital at a time of mounting confrontation between Islamists and the military-backed government.
The blasts further hike tensions a day before the third anniversary of Egypt's 2011 uprising that toppled autocrat Hosni Mubarak - when supporters of the military and their Islamist opponents have each vowed rival rallies in the streets to press their cause.
There was no immediate claim of responsibility for Friday's morning attacks. Islamic militants have increasingly targeted police and the military since the July 3 coup against Mohammed Morsi and the ensuing crackdown by security forces against his Muslim Brotherhood and other Islamist supporters that has arrested thousands.
Authorities have branded the Brotherhood a terrorist group, accusing it of involvement in the militant violence. The Brotherhood has denied any link. But the branding has helped fuel a wave of popular sentiment against the group and in favor of the military.
Islamists are trying to use Saturday's anniversary to build momentum in their campaign of protests to "break the coup." Military supporters, in turn, aim to show broad popular support for the government and military chief Gen. Abdel-Fattah el-Sissi, the man who ousted Morsi.
Interior Minister Mohammed Ibrahim, who is in charge of police, called the bombings a "vile terrorist act" aimed at spreading panic ahead of Saturday's pro-military rallies. "But people will only increasingly insist ... and join the masses in millions" on Saturday, he told reporters at the site of the bombed police headquarters.
The office of interim President Adly Mansour vowed in a statement after the attack that it is determined to "uproot terrorism" and said it could be forced to take "exceptional measures." It did not elaborate.
On its Twitter account, the Muslim Brotherhood posted a message in English condemning the "cowardly bombings in Cairo, express condolence to families of those killed and demand swift investigation." There was no similar one in Arabic.
Friday's violence began around 6:30 a.m. when a suicide car bomber blasted the capital's main police headquarters in a downtown square, killing at least four people and sending billows of black smoke into the sky. The Health Ministry said in a statement that four policemen were killed and nearly 50 people wounded.
Several police officers sat on the sidewalk weeping outside the building as ambulances rushed in. A blanket covered a corpse on the ground that officers said was the suicide bomber.
The blast dug a large crater into the pavement, and the street was littered with vehicle parts, shattered glass, bricks and stones. The seven-story facade of the security headquarters was wrecked, with air conditioning units dangling from broken windows. A nearby courthouse and shops were also damaged.
"Execution for Morsi and his leaders," one man shouted through a megaphone as a crowd gathered around the wreckage chanting against the Brotherhood.
Ibrahim said the attack was carried out by two men in a pick-up truck. The vehicle stopped outside the building, one man got out, apparently when a policeman halted the truck, and then the other man detonated the blast, he said.
The explosion also heavily damaged the renowned Museum of Islamic Art, on the other side of Bab el-Khalq Square. Windows were blasted out all the way up the facade of the building, which was built in 1881 and recently underwent a multimillion-dollar renovation. The antiquities minister, Mohammed Ibrahim, said artifact inside were damaged, including a rare collection of Islamic art objects and that the museum will have to be rebuilt.
About two hours later, another bomb struck a police car on patrol near a metro station in the capital's Dokki district on the other side of the Nile River, killing one person and wounding eight others, security officials said.
At the site of the attack, a stain of blood on sidewalk was seen next to shattered glass after the blast broke windows of near building.
A third, smaller blast targeted the Talbiya police station about four kilometers (two miles) from the famous Giza Pyramids but caused no casualties, the officials said. They spoke on condition of anonymity because they were not authorized to speak to the media.
Ahmed Ghaith, a retired army officer who witnessed the blast, said he was waiting for a bus when the blast tore down an advertisement placard and dirt spread everywhere.
"No one killed or injured, not even a cat. We know ... they will get nothing at the end," referring to the Muslim Brotherhood group typical suspect despite its repeated denials.
The attacks came a day after the country's military and security leaders marked Police Day depicting security forces as national heroes battling terrorism.
The most prominent attacks were 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.
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.
A 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.