The strike raised fears of a militant campaign of revenge for the coup and the likelihood of an even tougher hand by authorities against protesters demanding Mohammed Morsi's return to office.
The interim president compared the attack to the insurgency waged by Islamic militants in the 1980s and 1990s against the rule of now-ousted autocrat Hosni Mubarak, when militants carried out numerous assassination attempts, killing the parliament speaker. Mubarak himself survived an assassination attempt in 1994, when militants attacked his convoy in Addis Ababa, Ethiopia.
That insurgency provided Mubarak with a justification for a nationwide state of emergency, lifted only after he was driven from power by an uprising in 2011.
Since Morsi's ouster in a July 3 coup, Egypt has been back under emergency law, and police have arrested nearly 2,000 members of his Muslim Brotherhood and other Islamist supporters.
In mid-August, authorities forcefully dispersed two pro-Morsi sit-in camps in Cairo after days of warnings, setting off violence that killed hundreds nationwide. The move led to retaliatory strikes on government buildings, police stations and churches around the country.
Islamic hard-liners have since stepped up attacks on security forces in the Sinai Peninsula and in the south, and have increasingly brought attacks to the capital.
Still, Thursday's bombing against Mohammed Ibrahim, who heads the police force waging the crackdown, was a substantial escalation. There was no immediate claim of responsibility.
Interim President Adly Mansour's office vowed it would "not allow the terrorism the Egyptian people crushed in the 1980s and 90s to raise its ugly head again." Military leader Abdel-Fattah el-Sissi, the man who ousted Morsi, pledged to continue the fight against terrorism.
Egyptian media have for weeks vilified the protesters, blaming the violence on Morsi's supporters and a terrorism campaign. After Thursday's attack, state media urged citizens to exercise caution, report suspicious activities or individuals, and called on authorities to widen their crackdown on suspected terrorists.
The attack is likely to further isolate the Islamists. Liberal politician Amr Moussa called on the ousted president's backers to take a clear position against the bombing.
"When lives of innocents are targeted, those who support that or justify it will not be accepted among us," said Moussa, who sits on a newly appointed constitutional panel.
Morsi's supporters sought to distance themselves from the attack.
The Anti-Coup Coalition, a group of Islamist factions that has spearheaded protests since Morsi's ouster, predicted it would be used as a pretext for widening the crackdown on its opponents.
"The coalition is against any violent act, even if it is against those who committed crimes against the people," the group said. "It expects that such incidents will be used to extend the state of emergency and to increase the use of oppression, repression and detention which have been used by the coup authority."
The group vowed to keep up the protests demanding Morsi's reinstatement and called on supporters to prepare for rallies on Friday.
The bomb was detonated in the late morning as Ibrahim's convoy passed through Nasr City, an eastern district of Cairo where Morsi's supporters have held near daily protests.
Security officials said initial investigations showed the blast came from a parked car with about 90 pounds (40 kilograms) of explosives packed in its trunk. They spoke on condition of anonymity because the probe was still underway.
At least 10 policemen and 11 civilians were among the wounded, including a 7-year-old boy who lost his right leg, the officials said.
A mangled body was found near the vehicle and investigators were trying to determine if it was a bystander, a bomber or a lookout tasked with alerting the convoy's approach, the officials said.
Several vehicles in Ibrahim's convoy - including his own - were damaged, but he was not hurt. The blast left a main avenue in Nasr City strewn with charred vehicles as heavy black smoke rose overhead. Nearby shop fronts were mangled and windows of nearby apartment buildings were shattered.
Two of Ibrahim's guards were seriously injured, according to Lt. Col. Emad Hamad, who spoke from the hospital to private TV station ONTV.
Hamad, who was part of the minister's security detail, said the explosion hit the first car in the convoy, badly burning a policeman and an officer next to him. Two women and a child were also badly injured, and all lost a leg in the explosion.
The father of Fares Hegazy, the 7-year old victim, said the explosion came from a car double-parked on the main road the convoy was traversing.
Just before the attack, men on a motorcycle snatched a woman's purse, setting off a commotion that sent security forces chasing the motorcycle, he told ONTV.
Appearing shaken, Ibrahim told state television his black SUV was directly hit.
"Even if I am martyred, another interior minister will come and continue the war on the evil terror until we secure the country," he told reporters at the Interior Ministry in central Cairo.
Ibrahim, who has aggressively led the crackdown on Islamists, said in a television interview last week that he had received death threats.
He was appointed to his post by Morsi and came under sharp criticism at the time even by some in the police as too beholden to the Islamist president. But since the coup, he has fully embraced the new military-led leadership.
Nearly 2,000 members of Morsi's Muslim Brotherhood and other Islamist groups have been detained over the past month, most accused of inciting violence or weapons possession. Most deny the charges.
Morsi, who has been held in an undisclosed location since his ouster, has been referred to trial for inciting the murder of his opponents while in office.
Some of Morsi's more hard-line supporters have publicly threatened to wage a campaign of assassinations and car bombings against officials of the military-backed government until the former president is reinstated.
Security officials have said they have unraveled hit lists that included politicians, public figures and journalists.
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