Egypt's military-backed rulers declared the Muslim Brotherhood of ousted president Mohammed Morsi a "terrorist" organization Wednesday, blaming it for a deadly police headquarters bombing already claimed by an al-Qaida-inspired group.

The decision is likely to accelerate a crackdown on the movement that has killed more than 1,000 people, mostly Islamists, in street clashes and seen thousands imprisoned since Morsi's overthrow by the military in July.

The Egyptian army said Wednesday it had foiled a plan by Palestinian Islamist movement Hamas to attack a strategic security building in North Sinai, where militants have increased activity in recent months.

A member of the movement revealed the plan when the army interrogated him, military spokesman Colonel Ahmed Ali said in a statement.

An al-Qaida-inspired group based in Egypt's Sinai Wednesday claimed a suicide car bombing of a police headquarters north of Cairo that killed 15 people, the deadliest such attack since Mohammed Morsi's overthrow.

The brazen assault underscored the military's challenge to contain Sinai militants who have killed more than 100 soldiers and policemen in a wave of attacks since the army ousted Islamist president Morsi on July 3.

The United Nations on Tuesday demanded the release of three Egyptian activists jailed for three years for organizing an unauthorized protest, a spokeswoman for the U.N. Human Rights Office said.

The convictions on Sunday are "of great concern", Ravina Shamdasani said, calling for the "immediate and unconditional release of all prisoners detained solely in connection with peaceful protests, unless the authorities have solid evidence (of) recognizable criminal offences."

Egyptian police arrested ousted Islamist president Mohamed Morsi's former prime minister Tuesday as tensions rose after a major car-bomb attack on a police building killed 15 people.

The interior ministry said ex-premier Hisham Qandil, who is facing jail after being convicted of failing to respect a court ruling while in office, was arrested in the desert outside Cairo with a "smuggler attempting to escape to Sudan".

Angry Egyptians standing on piles of debris and shattered glass pointed the finger of blame at the Muslim Brotherhood after a car bomb ripped through Mansoura's police headquarters on Tuesday.

"The people want the execution of the Muslim Brotherhood," shouted residents amid the wreckage of burnt-out cars and collapsed walls left by the pre-dawn blast, which peeled off a section of the building.

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 Mohammed 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 polarized over his ouster, with over 1,000 people killed in months of unrest.

Egypt's army said Monday that, since August, it has killed 184 "terrorists" in North Sinai, where militants have carried out nearly daily attacks against security forces after the toppling of president Mohammed Morsi.

The military has "eliminated 184 terrorists and radicals," state news agency MENA quoted army spokesman Colonel Ahmed Ali as saying.

More than 450 Muslim Brotherhood members imprisoned in Egypt launched a hunger strike Monday over their "inhuman treatment" after being jailed following the military's overthrow of president Mohammed Morsi, the group said.

A Twitter account operated by the Brotherhood, which has largely been driven underground by a massive crackdown, said prisoners have been "banned from family visits, legal counselling, medical care and (live in) overcrowded and unhygienic cells."

Egypt said Sunday a panel has been formed to probe the violence that has plagued the country since June 30, when millions demanded the resignation of Islamist president Mohammed Morsi.

Egypt has been rocked by militant attacks and street clashes since the army ousted Morsi on July 3 following massive protests against his year-long rule, leaving the country bitterly polarized.