A suspected suicide car bombing outside an Egyptian police headquarters killed at least 15 people Tuesday in one of the deadliest attacks since the military overthrew Islamist president Mohamed Morsi.
The explosion in a city north of Cairo, which military-installed authorities suggested was carried out by Morsi's Muslim Brotherhood, comes as the country is bitterly polarised over the president's ouster, with more than 1,000 people killed in months of unrest.
The Muslim Brotherhood, which publicly renounced violence decades ago, condemned the bombing.
Analysts said the attack was likely the work of more radical Islamists, who have carried out a string of similar attacks in the Sinai peninsula targeting security forces.
The interior ministry said at least 12 of the dead were policemen in the attack in the Nile Delta city of Mansoura.
A security official later said a police conscript was found dead in the rubble of the blast, and a police officer was discovered dead under the wreckage of the building.
The blast also killed a civilian, medics said.
Investigators suspect a suicide bomber rammed a car freighted with powerful explosives through barriers surrounding the building shortly after 1:00 am (2300 GMT Monday).
The explosion peeled off part of the building's facade and could be felt up to 20 kilometres (12 miles) away.
Casualties were rushed to a nearby hospital, where one witness told AFP the blast had sent him flying through the air.
"I heard a massive explosion behind me. I flew through glass and wreckage and then lost consciousness," he said from his stretcher, his head swathed in bandages.
The bombing was the deadliest since Morsi's overthrow outside of the Sinai Peninsula bordering Israel and Gaza, where militants have killed more than 100 police and soldiers.
Hours after the bombing, prime minister Hazem Beblawi labelled the Brotherhood a "terrorist" group but did not blame the organisation for the blast.
Later, thousands turned out at a funeral for the victims, whose corpses, shrouded in white and Egyptian flags, were carried on fire engines through chanting crowds.
Ahmed Fouad, a policeman who lost two comrades in the attack, said "I go to work every day not knowing whether I'll return."
Crowds torched a bus belonging to a tourism company, saying its employees had made pro-Islamist salutes, and then ransacked a supermarket believed to belong to Muslim Brotherhood sympathisers.
"The Muslim Brotherhood is an international terrorist organisation. They are responsible for what happened in Mansoura," said teacher Hamada Arafat.
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"They are now adopting tactics like Al-Qaeda."
Interior minister Mohamed Ibrahim said the explosion was meant to intimidate voters ahead of a constitutional referendum next month billed as the first step in the democratic transition after Morsi's overthrow, to be followed by fresh elections.
"This is an attempt to terrorise people because of the referendum. There is a security plan, and this will not affect the referendum," he told reporters after surveying the damage.
It was the second time the building has been targeted this year, after a pipe bomb outside killed a policeman in July.
Al-Qaeda-inspired groups on the peninsula have claimed attacks, including a bombing aimed at the interior minister in September outside his Cairo home.
Shadi Hamid, research director at the Brookings Doha Center, said "this is a more sophisticated attack than previous ones. It could be a sign of things to come... The insurgency in Sinai is becoming emboldened and extending attacks outside the Sinai."
Morsi and other Islamists face trial for having allegedly colluded with militants to launch attacks in the country.
Brotherhood condemns bombing
The interim presidency declared three days of mourning after the explosion, and said the security forces would "avenge our martyrs."
"Our war on terrorism has become the first priority of this period," a presidency statement said.
The Brotherhood condemned the bombing "in the strongest possible terms," while The Anti-Coup Alliance led by the Brotherhood said the "military coup is responsible for the continuation of such sinful bombings."
And US State Department spokeswoman Jen Psaki condemned what she called a "terrorist attack."
The Brotherhood, Egypt's most well-organised opposition movement during decades of dictatorship, prevailed in polls following the overthrow of longtime dictator Hosni Mubarak in 2011.
Morsi became Egypt's first freely elected leader after winning elections last year, but was removed from power after massive protests against his turbulent rule.
More than 1,000 people have been killed in a subsequent crackdown, including hundreds on August 14 in Cairo, when police stormed two pro-Morsi protest camps.
More than 2,000 Islamists have been arrested since Morsi's overthrow, virtually the entire top leadership of the Muslim Brotherhood.
Police on Tuesday arrested Morsi's former prime minister Hisham Qandil.
He had been sentenced to a year in prison for ignoring a court order to renationalise a company that had been privatised.