A powerful blast tore through a police headquarters in an Egyptian Nile Delta city early Tuesday, killing 13 people, wounding more than 100 and leaving victims buried under rubble in the deadliest bombing yet in a months-long wave of violence blamed on Islamic militants.

Officials sought to pin the ultimate blame for the bombing on the government's top political nemesis, the Muslim Brotherhood, which has been leading a campaign of protests since the July ouster of Islamist President Mohammed Morsi. The Brotherhood in turn accused the government of trying to scapegoat it to justify intensifying a crackdown.

At the funeral for the 12 policeman and one civilian killed, hundreds massed in a main square of the city of Mansoura where the bombing took place, chanting, "The people want to execute the Brotherhood." They raised posters reading "no to terrorist groups" and pictures of military chief Gen. Abdel-Fattah el-Sissi, who removed Morsi and is the country's most powerful figure.

Egypt has seen an escalating campaign of spectacular bombings and gun attacks, mainly against security forces, since the military ousted Morsi and launched a fierce crackdown on his Muslim Brotherhood. Most attacks have been centred in the Sinai Peninsula, where multiple militant groups operate, but the insurgency has been spreading to the heavily populated Delta and the capital, Cairo.

The military-backed interim government has sought to portray the Brotherhood as largely responsible for violence — though authorities have presented no evidence. A government panel was meeting Tuesday and Wednesday to discuss declaring the group officially as a "terrorist organization."

Military seeks to sully Brotherhood's name

Such a move would further tarnish the group before Egyptians vote in a Jan. 14-15 referendum on a revised constitution, a key step in the military-backed transition plan. Morsi's supporters oppose the new document, which amends the constitution passed under his rule. But the interim government is pushing for its overwhelming passage to show the legitimacy of the military's ouster of Morsi and the new political system.

Last week, prosecutors referred Morsi and other top Brotherhood leaders to trial on charges of organizing a large terrorist conspiracy, working with Hamas, Hezbollah, Iran and other militant groups and orchestrating the Sinai insurgency to avenge his ouster. Morsi supporters and rights groups have called the accusations implausible.

Egypt injured

Egypt Health Minister visits a boy injured in the bombing. (Mohamed Abd El Ghany/Reuters)

Interim Prime Minister Hazem el-Beblawi stopped short of directly blaming the Brotherhood for the attack, which he called the "worst kind of terrorism" against the state. But he grouped it in with the pro-Morsi protests as part of a string of "violations of the people's security."

He called the attack a "maximum offence" to Egypt and will be dealt with decisively and according to the law, without elaborating. El-Beblawi said his government has been working to implement a court order in late September banning the Brotherhood.

One of his spokesmen, Sherif Shawki, went further, accusing the Brotherhood, which he said showed its "ugly face as a terrorist organization, shedding blood and messing with Egypt's security," according to the state news agency MENA.

The attack on the security headquarters in Mansoura — a provincial capital 110 kilometres north of Cairo that is considered a stronghold for the Brotherhood — was the first major bombing in the Nile Delta. The same building had been targeted in July, when an explosive planted outside killed a policeman and wounded another.

Blast damages bank, theatre

Tuesday's blast at 1:10 a.m. local time brought down an entire section and side wall of the five-floor building. Dozens of parked cars were incinerated, and several nearby buildings were damaged, including a bank and theatre. Associated Press video from the scene showed bulldozers clearing the rubble.

An unidentified senior security official told MENA that a pick-up truck laden with a large amount of explosives is suspected to be behind the attack. He said investigators are still looking to see whether it was detonated by timer or remote control.

But another official said the bomb may have been planted outside the building, saying no traces of a bomb vehicle or the explosive device itself had been found yet. The official spoke to The Associated Press on condition of anonymity because he was not authorized to speak to reporters.

The dead included 12 policemen, including two officers, and one civilian, according to a police statement. Health Ministry spokesman Mohammed Fatahallah said 101 people were wounded. Among the injured were the city's security chief — who lost an eye — and his assistant, the state news agency MENA reported. Most of the victims were policemen, many of whom were buried beneath the debris.

No one immediately claimed responsibility for the bombing.

A day earlier, an al-Qaida-inspired group that has carried out multiple suicide bombings and other attacks in Sinai — Ansar Beit al-Maqdis, or the Champions of Jerusalem — threatened more attacks on the military and police, saying it considers Egyptian troops to be infidels because they answer to the secular-leaning military-backed government.