MANSOURA, Egypt (AFP) - A powerful car bomb tore through an Egyptian police headquarters Tuesday, killing at least 14 people, in one of the worst attacks since the army ousted Islamist president Mohamed Morsi.

The military-installed government said the bombing in Mansoura city, north of Cairo, was aimed at derailing Egypt's democratic transition, and immediately suggested Morsi's Muslim Brotherhood was behind it.

But the Islamist movement, which says it renounced violence decades ago, condemned the devastating bomb attack whose impact was felt as far as 20 kilometers away.

The explosion, which peeled off part of the building's facade, was the deadliest since Morsi's ouster in July outside the Sinai Peninsula, where militants have killed more than 100 policemen and soldiers.

Hours after the bombing, Prime Minister Hazem Beblawi labelled the Brotherhood a "terrorist" group, though he did not blame the organization for the blast.

The explosion ripped through the multi-storey police headquarters shortly after 1:00 am, killing at least 14 people and wounding more than 100, officials said.

"The majority of the casualties are from the police," said Omar al-Shawatsi, the governor of Daqahleya, of which Mansoura is the capital.

Daqahleya security chief Sami El-Mihi was wounded and two of his aides were killed, security officials said.

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 wreckage of an armored police car was nearby, while at least 10 civilian vehicles were damaged and a nearby building in rubble.

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.

"This is an attempt to terrorize people because of the referendum. There is 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 the headquarters killed a policeman in July.

Angry residents of the area vented their fury on the Brotherhood.

"The Muslim Brotherhood is an international terrorist organization. They are responsible for what happened in Mansoura," said Hamada Arafat, a school teacher.

"They are now adopting tactics like Al-Qaeda."

Prosecutors and officials say the Brotherhood has links with militants who have stepped up attacks on security forces across Egypt, but mainly in the Sinai.

Al-Qaeda-inspired groups on the peninsula have claimed numerous attacks, including a bombing aimed at the interior minister in September outside his Cairo home.

"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," said Shadi Hamid, research director at the Brookings Doha Center.

Beblawi said the bombing was an attempt to thwart steps towards democracy, and declared the Brotherhood a terrorist group.

"Prime Minister Beblawi has declared the Muslim Brotherhood as a terrorist organization," state media quoted his spokesman as saying.

"Terrorism will not succeed in derailing the implementation of the road map," the premier was quoted as saying later, without mentioning the Brotherhood by name.

The constitution, if approved in January's referendum, is to be followed by parliamentary and presidential elections by mid-2014.

The Brotherhood condemned the bombing "in the strongest possible terms",  and also lashed out at Beblawi.

"It is no surprise that Beblawi, the military junta's puppet prime minister, has decided to exploit the blood of innocent Egyptians through inflammatory statements designed to create further violence, chaos and instability," it said.

The 85-year-old movement prevailed in a series of polls following the overthrow of longtime dictator Hosni Mubarak in 2011, and Morsi became Egypt's first freely elected leader after winning elections last year.

Morsi was removed from power after massive street protests against his turbulent one-year rule, with millions accusing him of power-grabbing and economic mismanagement.

More than 1,000 people have since been killed in a crackdown, including hundreds on August 14 in Cairo when police stormed two protest camps of his Islamist supporters.

The crackdown has seen thousands of Islamists, including the Brotherhood's  entire leadership, arrested.