Deadly bombings hit the Daqahliya Security Directorate in the early hours of Tuesday morning  in the Nile Delta city of Mansoura
The blast killed 12 and injuring 134 in what appears to be the worst terrorist attack on a government site  since the ouster of Islamist president Mohamed Morsi last July.
Eight policemen, a civilian and three unidentified individuals were among the dead, Egypt's Interior Ministry said in a statement.
The explosion ripped through the building's side façade and damaged a number of police vehicles and parts of adjacent buildings which include the city's council building, a state-owned theatre and a bank.
The head of Mansoura's Security Directorate was among the injured.
Al-Ahram's Arabic correspondent in Mansoura said streets in the city of 500,000 residents became calmer after sunrise as thousands of citizens helping with rescue operations went home after victims of the blast were transferred to area hospitals.
Thousands of citizens heeded official calls to donate blood in area hospitals.
However, doctors began turning away potential donors as blood banks acquired large quantities of blood.
The wreckage of an armoured police car could be seen nearby, while at least 10 civilian cars were damaged and a nearby building completely collapsed, said an AFP correspondent at the scene.
Furious reactions, denials
Angry residents of the area vented their fury on Morsi's Brotherhood movement. "The Muslim Brotherhood is an international terrorist organisation. They are responsible for what happened in Mansoura ," said Hamada Arafat, a school teacher to AFP.
"They are now adopting tactics like Al-Qaeda."
Prosecutors and officials say the Brotherhood has links with Islamist militants  who have stepped up attacks on security forces across Egypt especially in the restive Sinai Peninsula.
"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.
The Muslim Brotherhood was quick to deny any connection to the attack.
"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, an emailed statement from the group's London press office read.
The pro-Morsi National Alliance to Support Legitimacy has also condemned what it labeled a "criminal incident" that aims to provoke strife, asserting its peacefulness and denouncing all forms of bloodshed, reported the Brotherhood's Freedom and Justice Party on its website.Investigations are underway to identify the causes of the blast.
Cause of blasts
The first bomb, he said, was planted in a higher floor in the building, the second in a car next to the Security Directorate.
A third bomb planted in another car was defused, the source added.
However, minister of interior statements have not specified the sources of the blasts.
Egypt's Prime Minister Hazem El-Beblawi  vowed to hunt down the perpetrators of the explosion. He said the attack seeks to obstruct the roadmap drawn up by the country’s interim rulers following Morsi's July ouster by the army amid nationwide mass protests against his rule.
“This is an act of terrorism that aims at frightening the people and obstructing the roadmap. The black hands behind this act want to destroy the future of our country,” Beblawi told Egyptian satellite channel ONTV.
"The state will do its utmost to pursue the criminals who executed, planned and supported this attack," he asserted.
Beblawi refused to confirm that Egypt had designated the Muslim Brotherhood a terrorist group , contradicting an earlier statement by one of his aides.
Cabinet Spokesman Sherif Shawqi had earlier blamed the blast on the Muslim Brotherhood group of ousted Islamist president Morsi, and said Prime Minister Hazem El-Beblawi had officially declared the group a terrorist organisation.
“Whoever is behind this act is a terrorist and will be brought to justice and punished according to the law. But I do not wish to anticipate events," Beblawi added.
The final stages of Egypt's roadmap will begin when a referendum on the newly-drafted constitution, scheduled for mid-January, takes place. It will be followed by parliamentary and presidential elections.