CAIRO — A powerful bomb that may have been placed inside a booby-trapped truck ripped through a police headquarters north of the capital early Tuesday, killing at least 13 people in an attack that highlighted the Egyptian government's faltering struggle against a campaign of assassinations and bombings by militants.

The bombing, in the Nile Delta city of Mansoura, appeared to be one of the most brazen attacks since the military ouster of the president, Mohamed Morsi, in July. Television footage showed buildings stripped of their facades, mounds of rubble and the police headquarters crippled, with a jagged scar running across its five stories.

At least eight of the 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.

The attack, the second on the headquarters since July, renewed doubts about the government's ability to provide security just weeks before millions of Egyptians are expected to vote in a referendum on a draft constitution. And it seemed certain to strengthen a government crackdown that has been focused primarily on Mr. Morsi's Islamist movement, the Muslim Brotherhood, but has also lately swept up non-Islamist activists who have been critical of the government's policies.

At funerals in Mansoura for the bombing victims, hundreds of people awaiting the bodies held posters of Egypt's de facto leader, Gen. Abdul-Fattah el-Sisi, the powerful military chief, while chanting angry slogans against the Islamists.

"The people want the execution of the Brotherhood!," one chant went.

There were no immediate claim of responsibility. Officials gave conflicting accounts about the source of the explosion. An unidentified security official was quoted by the website of Al-Ahram, the flagship state newspaper, as saying that the source was a booby-trapped truck parked near the headquarters, on a side street where only police cars were permitted to park. But the head of the explosives department at the Ministry of Interior said officials were still removing rubble to determine the source.

A government spokesman initially blamed followers of the Brotherhood, calling it a terrorist group. In a statement, the Brotherhood press office in London condemned the bombing, calling it a "direct attack on the unity of the Egyptian people."

The attacks started after the military takeover in July, targeting soldiers and police officers, mainly in the relatively lawless Sinai Peninsula. The bombing on Tuesday appeared to be the deadliest to take place outside of the Sinai, raising fears of a broadening insurgency.

A previously obscure militant group called Ansar Beit al-Maqdis, which is not affiliated with the Brotherhood, has claimed responsibility for several of the most spectacular recent attacks on security personnel, including the attempted assassination of the 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 on Sunday, the group urged Egyptians to stop serving in the army or police. Those who did not, the statement said, "have themselves to blame."

Militants stepped up their attacks on the security services in August, after soldiers and police officers killed hundreds of Islamist protesters during the clearing of two Cairo sit-ins. Since then, 163 police officers have been reported killed in drive-by shootings bombings and other attacks.

In a news conference on Tuesday afternoon, the interim prime minister, Hazem el-Beblawi, cast the attack as one in a long series of "operations" against the state that included the assassinations of police officers, but also street protests.

"We will show no leniency to anybody," he said.