The attacks fueled fears of an increasing militant insurgency in retaliation for the military's July 3 ouster of Islamist President Mohammed Morsi and the subsequent crackdown on his Muslim Brotherhood and other Islamists.
The hours after the bombings saw a public backlash against the Brotherhood, which the government accuses of being behind the monthslong wave of bombings and shootings, though it denies any link. Angry residents joined in with security forces in clashes with Brotherhood supporters holding their daily protests in several districts of the capital and multiple cities across the country in violence that left eight dead.
In one Cairo neighborhood, pro-Morsi protesters clashing with security forces set fire to a police kiosk, sending a pall of smoke in the air. In the Mediterranean city of Alexandria, residents throwing stones and firing rounds of birdshot killed one Brotherhood supporter when they attacked Islamists marching after the funeral of a student protester killed the day before.
As police drove back from clashes with Brotherhood supporters in the capital's Giza district in the afternoon, they were hit by the day's fourth bombing - a roadside explosive that killed one policeman and wounded four others on Haram Street, a main avenue leading to the famed Giza Pyramids.
The turmoil intensified an atmosphere of chaos and fear on the eve of the third anniversary of the Jan. 25 start of the 2011 uprising that ousted Egypt's longtime autocrat, Hosni Mubarak. Amid the bombings and clashes, security forces closed major avenues in Cairo and sealed off central Tahrir Square and other city squares.
Three years since the uprising in the name of democracy, Egypt is locked into the fight between the new government and the Brotherhood, which gained political dominance in the first post-Mubarak elections only to lose it when massive public protests against them prompted the coup. The government installed since by the military has waged a fierce crackdown on the group, arresting thousands of its members and killing hundreds as police put down their protests demanding Morsi's return.
Authorities accused the Brotherhood as being behind the tide of militant attacks, branding it officially as a terrorist organization. The Brotherhood has called the accusation baseless, saying authorities are only trying to justify their drive to crush the group. Still, even some who dismiss the claims the group organized the insurgency fear that the crackdown is driving desperate young Brotherhood supporters into militant violence.
In any case, the branding has helped fuel a wave of popular sentiment against the group and in favor of the military among a public fearful of the militant attacks, which have largely targeted security forces but increasingly in public areas causing civilian casualties. After Friday's blasts, private TV stations aired called from listeners demanding that army chief Gen. Abdel-Fattah el-Sissi, the man who ousted Morsi, now act decisively to crush the Brotherhood.
Outside the Cairo security headquarters hit in Friday's first bombing, a crowd chanted slogans against the Brotherhood and in support of el-Sissi.
"Execution for Morsi and his leaders," one man shouted through a megaphone. A woman held up a picture depicting the Brotherhood as sheep, screaming, "Morsi is the butcher and el-Sissi will slaughter him."
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, but the turmoil raises the potential for the partial return of a curfew imposed on much of the country for several months after the crackdown on Morsi's supporters escalated in mid-August.
In a statement, the Brotherhood condemned the attacks and suggested the security forces themselves were behind it to justify an even wider crackdown. It said the bombings were "a prelude to unjust, dangerous decisions."
Touring the bomb site, Interior Minister Mohammed Ibrahim, who is in charge of police, called the bombings a "vile terrorist act" and implicitly blamed the Brotherhood, without naming it. "They will reach a point where coexistence will be impossible," he said.
Security officials later said three suspects had been identified as behind the security headquarters attack, saying they belonged to the Brotherhood and "extremist groups." The officials spoke on condition of anonymity because they were not authorized to release the information.
Friday's violence began around 6:30 a.m. when a vehicle packed with explosives went off outside the Cairo police headquarters in downtown Bab el-Khalq Square, killing four people and wounding 50, according to the Health Ministry.
As ambulances rushed in, several police officers sat on the sidewalk weeping outside the building, its seven-story facade shattered by the blast, with air conditioning units dangling from broken windows. The explosion dug a deep crater into the pavement, and the street was littered with vehicle parts, shattered glass, bricks and stones.
Authorities initially characterized the attack as a suicide bombing, but later investigations suggested the bombers may have escaped before the blast. The chief prosecutor said a pick-up truck laden with explosives stopped outside the headquarters' main gate and security cameras showed its driver jumping out, getting into a second car and driving away before the detonation.
Abdullah el-Sayyed, a 26-year-old salesman who lives behind the headquarters, said he was woken up by the blast, followed by heavy gunfire. He described policemen in panic.
"They were devastated. They were firing their guns in panic as if to call for rescue," he said.
He said he plans to return to his home village in Fayoum south of Cairo because he no longer feels safe. "It's not worth it anymore to stay here. Every day I ride the metro and go past here," he said.
The explosion damaged a nearby courthouse and shops as far as 500 yards away. The reknowned Museum of Islamic Art, housed in a 19th Century building on Bab el-Khalq Square that has just completed a multimillion-dollar renovation, was heavily damaged: Windows of the facade were shattered, some ceilings inside collapsed and broken pipes sprayed water inside.
The antiquities minister, Mohammed Ibrahim, said some of the museum's collection of rare Islamic art objects was damaged and the museum would 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.
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.
The wave of militant attacks since Morsi's fall has largely targeted the military and police, though civilians have also been killed. The most prominent pervious 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.
An al-Qaida-inspired group, Ansar Beit al-Maqdis, or the Champions of Jerusalem, has claimed responsibility for most of the attacks, saying they aimed to avenge the killings of Morsi's supporters in the post-coup crackdown. On Thursday, the group issued an online audio statement warning police and soldiers to defect or else face new attacks.
The violence comes as both the Brotherhood and military supporters are gearing up to hold rival rallies on Saturday's anniversary.
Islamists are trying to use the day 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 el-Sissi, whose backers are calling on to run for president.
The fear of militant insurgency is likely to stoke support for the military and security forces among a public that have rarely seen bombings in the streets of their cities.
After the security headquarters blast, a nearby resident, Mostafa Mohammed, walked in the street with a picture of el-Sissi on his chest, denouncing those behind the attack as "traitors."
The 66-year-old said that when he was awoken by the blast and hugged his daughter. "I thought it was judgment day."
Associated Press journalists Laura Dean, Maamoun Youssef, Mamdouh Thabit and Khalil Hamra contributed to this report.