CAIRO (AFP) - A spate of deadly bombings put Egyptian police on edge on Saturday, as supporters and opponents of the military-installed government prepared rival rallies for the anniversary of the 2011 Arab Spring uprising.

Hours before the rallies were due to start, residents of a north Cairo neighbourhood woke up to a small explosion outside a police training centre, a day after four blasts, including a car bombing outside police headquarters, killed six people.

An Al-Qaeda inspired group - Ansar Beit al-Maqdis, or Partisans of Jerusalem - claimed responsibility for the bombings, all of them targeting police, and urged ordinary Egyptian "Muslims" to stay away from police buildings.

Police deployed across the capital as supporters of ousted Islamist president Mohamed Mursi, toppled by the military in July, readied counter-demonstrations to the commemorations called by the authorities. The Islamists have announced more than a dozen planned marches from Cairo mosques for Saturday to launch 18 days of protests.

On Friday, clashes pitting Mursi supporters against their opponents and police killed 15 people nationwide, the health ministry said.

Police, who have killed hundreds of Islamist protesters in street clashes since Mr Mursi's overthrow, have vowed to put a stop to their planned demonstrations. But they have encouraged Egyptians to turn out in support of the interim government, and some politicians called for rallies to back army chief Abdel Fattah al-Sisi, the general who overthrew Mr Mursi in July.

Before midday, hundreds of pro-government demonstrators had already gathered in Tahrir Square, epicentre of the popular revolt that toppled autocratic president Hosni Mubarak in 2011. Tanks stood guard at the entrances to the square as some of the demonstrators chanted: "No to terrorists." Western governments warned their citizens of the risks of violence during the day.

The United States embassy urged nationals to "limit their movements... to the near vicinity of their neighbourhoods". Britons were urged to "stay inside", while Germany called on tourists to remain in their hotels.

Officials in the government and military have been hinting for days that the turnout at the pro-government rallies on Saturday could be a bellwether for a run by General Sisi in a presidential election promised for later this year.

Gen Sisi is widely seen as a strongman who can restore order and fight militancy, which the interim government blames on Mr Mursi's Muslim Brotherhood. The Brotherhood condemned Friday's bomb blasts, as they have previous attacks on the police and army.

But following a previous attack on a police building in December, also claimed by Ansar Beit al-Maqdis, the authorities declared the Muslim Brotherhood a "terrorist organisation", making even expressions of verbal support punishable by heavy prison sentences. That view has been taken to heart by the Brotherhood's opponents. After each of Friday's bombings, crowds gathered brandishing posters of Gen Sisi and shouting praise for the government he installed.

Mr Mubarak was forced to step down on Feb 11, 2011 after 18 days of demonstrations that left some 850 people dead, ending his three-decade rule of the Arab world's most populous country. On his overthrow, the armed forces took power, handing the reins over 16 months later to Mr Mursi - the country's first freely elected head of state.

But late last June, after just one year of turbulent rule by Mr Mursi's Muslim Brotherhood, millions of Egyptians took to the streets to demand his resignation. Three days later, Gen Sisi announced Mr Mursi's ouster. The Islamist president has been in custody ever since and is on trial in four separate cases.