A SUICIDE car bomb targeting Egyptian police headquarters and three other blasts in Cairo have killed six people, as militants step up attacks on the military-backed government.

Militant violence has been on the rise since the army overthrew president Mohammed Morsi of the Muslim Brotherhood group in July.

Security guards opened fire on a suicide bomber who tried to ram his car into the police headquarters in the capital. An explosion followed, killing four people and injuring 70 others, state media reported on Friday.

An al-Qaeda-inspired group said it had carried out the attack, which damaged the building and an adjoining Islamic arts museum in the area of Bab al-Khaleq, in medieval Cairo.

"The explosion was against the oppressive and unfaithful security forces,'' the Ansar Beit al-Maqdes, which is based in the restive Sinai Peninsula, said in a statement.

Just heard what sounded like loud explosion, plume of black smoke seen from window in Mounira #Cairo #Egypt pic.twitter.com/cGhzxlhpRc — shadi rahimi شادي (@shadirahimi) January 24, 2014

It had previously claimed responsibility for several attacks against security forces since July, when the military deposed Morsi.

Egypt's military-backed government launched a deadly crackdown on the Brotherhood after the coup and declared it a terrorist group.

Hours after the first attack, a bomb exploded near a police car in the up-market Cairo quarter of Dokki, killing one person and injuring at least 14, security officials said.

A third bombing targeted a police station in the Giza Pyramids area near Cairo. It caused no casualties.

The bomb was hurled at the building from a bus, a security official said.

A fourth blast went off near a cinema complex, in Haram street, south of Cairo, killing one person and injuring seven others. Shortly after, police defused another bomb in the same area, state-run television reported.

Security has been beefed up at police stations and the Interior Ministry in Cairo.

The bombings come a day after five policemen were gunned down at a checkpoint in the city of Beni Sueif, south of Cairo.

Saturday marks the third anniversary of the start of the popular uprising that forced Mubarak to step down.

StateTV: Eyewitness: There were people with machine guns firing after explosive sound. All shop windows shattered. #Cairo #Egypt — Egyptocracy (@Egyptocracy) January 24, 2014

Morsi became Egypt's first democratically elected president in June 2012.

Supporters and opponents of the military have called for rival rallies to mark the anniversary, raising fears of violence.

@omertaher انا سمعته و شفته كده حد عارف حاجه؟ pic.twitter.com/NXObUFDArK — Mostafa Shaban (@MostafaShaban) January 24, 2014

In the latest sign of the air of intimidation against dissent, a court sentenced a blogger, Ahmed Anwar, to three months in prison on Thursday for "insulting the police'' and "misusing the internet'' over a video he posted on YouTube depicting policemen belly dancing - mocking police for recently giving an award to a well-known belly dancer.

The deputy Mideast-North Africa director of Amnesty International on Thursday called on Egyptian authorities to "change course and take concrete steps to show they respect human rights and rule of law,'' including release "prisoners of conscience".

Otherwise, "Egypt is likely to find its jails packed with unlawfully detained prisoners and its morgues and hospitals with yet more victims of arbitrary and abusive force by its police,'' said Hassiba Hadj Sahraoui.

Egypt's Foreign Ministry described the report as "tarnishing the facts'' and said the government respects human rights while it is engaged in "combating terrorism.''

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