The bombing, in the Nile Delta city of Mansoura, was Egypt’s deadliest since militants began a campaign of assassinations and other attacks against the security services in July, when the armed forces ousted Mohamed Morsi as president. Officials have repeatedly accused Mr. Morsi’s Islamist political movement, the Muslim Brotherhood, of orchestrating the attacks, but have provided scant evidence.
Officials said they believed a suicide bomber drove an explosive-laden car through security barriers around 1 a.m. on Tuesday. The explosion rattled windows on the city’s outskirts and led to the partial collapse of several buildings near the police headquarters, trapping officers and civilians in the rubble.
The attack was the second on the headquarters since July and renewed doubts about the military-backed government’s ability to provide security for the public — or its own officers — just weeks before millions of Egyptians are expected to vote in a referendum on a draft constitution. And it seemed certain to strengthen a security crackdown that has been focused primarily on the Brotherhood, but has also lately swept up non-Islamist activists who have been critical of the government’s policies.
Late Tuesday, the Interior Ministry announced the arrest of Hesham Qandil, who served as prime minister under Mr. Morsi and was one of the few remaining members of the former president’s inner circle still free. Mr. Qandil was convicted this year of disobeying a court ruling to nationalize a private company.
Though there was no claim of responsibility for the attack on Tuesday, government officials quickly pointed fingers at the Brotherhood. One government spokesman, Sherif Shawki, condemned the Brotherhood as a terrorist group that “sheds blood and tampers with Egypt’s security,” while offering no details on how the group may have been involved.
The interim prime minister, Hazem el-Beblawi, left open the question of direct responsibility, but he framed the bombing as the worst in a long series of attacks targeting the state, including assassinations but also the kind of street protests by the Brotherhood and others that the government has tried to stamp out.
“We won’t be lenient with anyone,” he said at a news conference.
In a statement, the Brotherhood press office in London condemned the bombing, calling it a “direct attack on the unity of the Egyptian people.” Another Brotherhood-affiliated group, the Anti-Coup Alliance, suggested, without offering any evidence, a conspiracy behind the attack that somehow involved one of Egypt’s most powerful businessmen.
The militant attacks started in July, targeting soldiers and police officers, mainly in the relatively lawless Sinai Peninsula, and intensified in August, after the authorities killed hundreds of Islamist protesters during the clearing of two Cairo demonstrations.
Since August, at least 163 police officers have been reported killed in drive-by shootings, bombings and other attacks.
A jihadist group called Ansar Beit al-Maqdis, which has criticized the Brotherhood for refusing to take up arms against the state, has claimed responsibility for several of the most spectacular recent attacks on security personnel. They include the attempted assassination of Interior Minister Mohamed Ibrahim in September and the killing last month of a senior security official who was responsible for investigating Muslim extremists.
In a statement released Sunday, the group urged Egyptians to stop serving in the army or the police. Those who did not, the statement said, “have themselves to blame.”
At least eight of the Mansoura bombing victims were police officers, and three bodies remained unidentified, according to the Interior Ministry. The director of security for the city of Mansoura was among the wounded.
There were reports of retaliatory violence in the city on Tuesday, including the burning of a law office belonging to a Brotherhood member, and the sacking of a building belonging to another.
Thousands of people attended the funerals for the victims, with many holding aloft pictures of Egypt’s powerful defense chief and its de facto leader, Gen. Abdul-Fattah el-Sisi, while voicing anger at the Islamists.
“The people want the execution of the Brotherhood!” people chanted.
Mohamed Ayman, an accountant who attended the funerals, said he was relying on General Sisi, who is repeatedly mentioned as a likely presidential candidate, to “cleanse the country from the Muslim Brotherhood killers.”
Another man interjected. “You’re here for the rights of the people who died, or to raise el-Sisi’s pictures?” he asked.
He was soon drowned out by a new chant. “Come down, Sisi, we want you to be our president!” they said.