There have been riots in the Egyptian city of Mansoura after the bombing of a police station that killed 14 people.

Police said on Tuesday three bombs had been planted before the explosion at the police station, two of which went off at almost at the same time. The third one, found in a car nearby, was defused.

They warned that the death toll from the bombing, which injured 150, could rise because more people might be trapped in the building.

Al Jazeera's Mohamed Fahmy, reporting from Cairo, said Ansar Beit al-Makdis, an armed group active in the Sinai peninsula, had claimed responsibility for bombing.

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The group, which is also known as Ansar Jerusalem, posted their statement on the Internet, Fahmy said.

Despite the group's claim, there were anti-riots in Mansoura on Tuesday afternoon.

Hundreds of people attacked shops and businesses and set two vans reportedly owned by the Muslim Brotherhood on fire, according to Al Jazeera's Mahmoud Sobky, reporting from Mansoura.

The blast had prompted a cabinet statement declaring the Muslim Brotherhood a terrorist organisation, although officials did not directly accuse the group of staging the attack.

The Brotherhood, which is already outlawed, condemned the bombing as "an attack on the unity of the Egyptian people".

Thousands attend funeral

Adly Mansour, Egypt's interim president, has declared three days of mourning and thousands attended a mass funeral in Mansoura, north of Cairo, on Tuesday.

Mohamed Ibrahim, Egypt's interior minister, visited victims in the hospital where he promised that the January referendum would go ahead.

"The security plans have been made and what happened will not affect the referendum process because it has its own comprehensive security plan," Ibrahim said.

He said four people had been arrested after admitting their involvement in the attack.

"The attacks are an attempt to create a diversion and to terrorise people because of the referendum," he said. "But I want to reassure people that there is a plan in place, in co-operation with the armed forces to protect all of the election centres at the highest level."

Sections of the five storey building in the Nile Delta city has collapsed after the blast and police evacuated surrounding buildings.

Source: Al Jazeera and agencies