A powerful car bomb explosion has rocked a police headquarters in an Egyptian city north of Cairo, killing at least 14 people and injuring 150, officials said.

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

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

Egypt's interior ministry said preliminary investigations showed it was a suicide bomber, driving a car loaded with explosives, who rammed through the barricades around the police station in Mansoura and then detonated the car, according to Al Jazeera's Gregg Carlstrom, who is reporting from Cairo.

The bombing set off a riot in Mansoura on Tuesday afternoon, with hundreds of people attacking a business establishment and setting on fire two vans reportedly owned by the Muslim Brotherhood, according to Al Jazeera's Mahmoud Sobky, who is reporting from Mansoura.

The state has already defeated terrorism in the 1990s, and will overcome it again. Adly Mansour, Interim Egyptian President

But the Brotherhood denied any involvement in the blast and condemned it in an emailed statement.

"The Muslim Brotherhood considers this act as a direct attack on the unity of the Egyptian people and demands an enquiry forthwith so that the perpetrators of this crime may be brought to justice," the statement said.

Thousands of people gathered on the streets of Mansoura during the funeral for the bomb victims.

Egypt's interim President Adly Mansour has declared three days of mourning for the victims.

"The state has already defeated terrorism in the 1990s, and will overcome it again," Mansour said, referring to a years-long insurgency that killed hundreds of people.

He said the "war on terrorism" has now become a priority, adding that the presidency "will not hesitate to take exceptional measure" to secure the country.

An interim government spokesman had earlier accused the Muslim Brotherhood of orchestrating the attack and branded the group a "terrorist organisation."

The Prime Minister Hazem el-Beblawi described the attack as a "terrorist incident," and vowed that the perpetrators "will not escape justice.''

"The incident we saw was the most heinous form of terrorism," Beblawi said.

Arrests made

Mohamed Ibrahim, Egypt's Interior Minister, said four people have been arrested after admitting their involvement to the incident.

"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 cooperation with the armed forces to protect all of the election centers 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.

The bombing comes weeks before Egypt is to hold a referendum on a new constitution, billed by the army-backed government as the first step towards democracy since the military toppled former president Mohamed Morsi in July.

Al Jazeera's Fahmy said five high-ranking police officers were among the dead and that that two other senior officers were critically injured.

"The explosion was so strong that parts of the gate surrounding the directorate are demolished," Fahmy said. "The injured are both police and civilians. Rescue teams are pulling people from under rubble."

Inside information

Earlier, Fahmy said information indicated that the attackers may have had inside information about who was at a meeting in the building.

Omar al-Shawatsi, the governor of Dakhalya, of which Mansoura is the capital, told state media all of the casualties were police.

"The explosion was caused by a car bomb," Shawatsi said.

The impact of the explosion was felt around 20 kilometres away and shattered the windows of nearby buildings, security sources said.

The Daqahleya region's head of security, Sami El-Mihi, was wounded in the blast and two of his aides were killed, security sources said.