After the blast, several police officers sat on the sidewalk outside the building, weeping as ambulances rushed in. A body lay on the ground, covered by a sheet as a crowd of distraught-looking residents surveyed the damage.
Touring the 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 in the security headquarters attack, adding that they belonged to the Brotherhood and "extremist groups." The officials spoke on condition of anonymity because they were not authorized to release the information.
About two hours after the Bab el-Khalq blast, attackers threw a bomb at a police car near a metro station in the Dokki district on the other side of the Nile River, killing one person and wounding eight others, the prosecutors' office said.
A third, smaller blast targeted the Talbiya police station about four kilometers (two miles) from the famed Giza Pyramids but caused no casualties, security officials said. They spoke on condition of anonymity because they were not authorized to speak to the media.
Hours after the attacks, the Brotherhood held its daily protests that they have vowed to step up ahead of Saturday's anniversary. The marches quickly turned into clashes with police, joined by residents furious at the Brotherhood, in several districts of Cairo and in cities across the country. The Health Ministry said 14 people were killed in the violence.
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 a day earlier.
As police drove back from clashes with Brotherhood supporters in the capital's Giza district, they were hit by the day's fourth bombing — a roadside explosive that killed one person and wounded four others.
Police and soldiers have been targeted by multiple attacks in recent months. In December, a suicide car bombing blasted the main security headquarters of a Nile Delta province, killing 16 people. The interior minister survived an attempted car-bombing assassination attempt in Cairo in September.
An al-Qaida-inspired group, Ansar Beit al-Maqdis, or the Champions of Jerusalem, has claimed responsibility for most 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 face new attacks.
The Islamist alliance grouping the Brotherhood and its allies condemned Friday's attacks and blamed them on the Interior Ministry, saying it wanted to turn the public against the Islamists.
It vowed to push ahead with protests Saturday, saying, "the revolution will continue down its peaceful track to bring down the military coup."
Government supporters are also planning giant rallies on Saturday to show their backing for the military — and to call for army chief el-Sissi to run for president.
Associated Press journalists Laura Dean, Maamoun Youssef, Mamdouh Thabit and Khalil Hamra contributed to this report.