Five people travelling in a bus in Cairo have been injured after a bomb went off at a traffic island, signaling the sliding of an open contest for political dominance between a secular military and Islamists into a violent tit-for-tat spiral.

The crude devise exploded—and another one was defused —after the devastating bombing on Tuesday of a building of the Security Directorate in Mansoura, a city 120 kilometers northeast of the capital. Sixteen people, mostly policemen were killed in the car bombing of the facility—a symbol of state authority. An al-Qaeda-inspired militant group Ansar Beit al-Maqdis (Champions of Jerusalem) claimed responsibility for the attack.

On Monday, the group warned the country’s military and police to desert their ranks or face death at the hands of its fighters. In a statement on militant websites, it labeled Egyptian troops as infidels because they belong to a "secular government". However, the government pinned the blame of the attack on the Muslim Brotherhood, a mainstream Islamist organisation, whose candidate, Mohamed Morsy had been elected President in the first elections after the removal of former head of state, Hosni Mubarak, in 2011. The Brotherhood has denied the charge.

Nevertheless, the Mansoura attack persuaded the military backed interim government on Wednesday to designate the Brotherhood as a terrorist organisation. Egypt’s Deputy Prime Minister Hossam Eissa, who announced the decision said, in reference to the Mansoura attack that Egypt was “horrified from north to south by the hideous crime committed by the Muslim Brotherhood group". He added: "It's not possible for Egypt the state or Egypt the people to submit to the Muslim Brotherhood terrorism."

On its part, the Brotherhood has vowed to continue its protests. "The protests will continue, certainly," said Ibrahim Munir, a member of the group's executive council who is in exile in London, according to AFP. He called the move by the government “illegitimate" and “an attempt to frame the Brotherhood”.

The latest designation intensifies the crackdown on the Brotherhood, which has included the toppling in July of Mr. Morsy, now in jail, during a military coup.

The recourse to force and harsh administrative measures seems to have played a part its driving dissent underground, especially after the Egyptian interim government last month banned protests that had not been pre-authorised by the provisional administration.

Thursday’s explosion took place inside Cairo’s Nasr city, which is being turned into a symbol of defiance by the Brotherhood and its supporters. The daily Al Ahram is reporting on its website that the nearby Al Azhar University has become the site of large anti-government protests, resulting in repeated clashes with the security forces. In November, 12 students from the university were sentenced to 17 years in prison for rioting at the institution’s headquarters.

After toppling Mr. Morsy, Egyptian security forces had, in August, stormed and uprooted an encampment in Nasr city of thousands of supporters of the deposed President, resulting in a bloodbath and serial arrests of the Brotherhood’s leaders.

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