A string of bombings hit police forces across Cairo on Friday, including a suicide car blast that ripped through the city's police headquarters and wrecked a nearby museum.

Five people were killed in the attack on the Egyptian capital at a time of mounting confrontation between Islamists and the military-backed government.

The blasts escalated tensions just a day before the third anniversary of Egypt's 2011 uprising that toppled dictator Hosni Mubarak.

Supporters of the military and their Islamist opponents have each vowed rival rallies in the streets.

Islamist militants have increasingly targeted police and the military since the July 3 removal of then-president Mohammed Morsi and a crackdown by security forces against his Muslim Brotherhood and other Islamist supporters that has led to thousands of arrests.

Islamists are trying to use today's anniversary to build momentum in their campaign of protests to "break the coup."

Military supporters, in turn, aim to show broad popular support for the government and military chief General Abdel-Fattah el-Sissi.

Interior Minister Mohammed Ibrahim, who is in charge of police, called the bombings a "vile terrorist act" aimed at spreading panic before today's pro-government rallies.

"But people will only increasingly insist ... and join the masses in their millions," he said at the site of the bombed police headquarters.

The office of interim President Adly Mansour vowed in a statement after the attack that it was determined to "uproot terrorism" and warned that it could be forced to take "exceptional measures."