An Egyptian masked policeman guards Cairo's state security chief, Osama al-Saghir, third right, as he visits Tahrir Square, the epicentre of the 2011 uprising.

Cairo - Demonstrators began gathering on Saturday in Egypt's Tahrir Square to mark the third anniversary of the start of its 2011 revolution, though streets remained empty elsewhere in a city on edge following a spate of bombings claimed by militants.

An al-Qaeda-inspired militant group claimed responsibility for bombings targeting police that killed six people Friday. A bomb exploded early on Saturday near a police training institute, but caused no casualties, officials said.

Those in Tahrir Square, the focal point of protests that toppled autocrat Hosni Mubarak, chanted anti-Islamist slogans and carried banners in support of a military that carried out a popularly supported July coup that toppled Islamist President Mohammed Morsi. Others carried posters supporting powerful Defence Minister Gen. Abdel-Fatah el-Sissi, already being talked about as a possible presidential candidate in coming elections.

“Come down (nominate yourself), oh Sissi,” the small crowd gathered there chanted. Armoured personnel carriers sat the entrances of the square and soldiers stood guard.

Radio stations aired patriotic songs Saturday as military helicopters flew over the square. Private television stations aired images of small rallies in different provinces.

However, streets largely looked empty around the capital after Friday's bombings. A statement issued by militant group Ansar Beit al-Maqdis, or the Champions of Jerusalem, claimed the attacks, promised more assaults and warned citizens to stay away from police stations.

“We tell our dear nation that these attacks were only the first drops of rain, so wait for what is coming,” the statement read. It was posted on two militant websites that have previously carried statements by the group and other al-Qaeda allies.

Early Saturday, a bomb exploded next to a police training institute in eastern Cairo, said Hani Abdel-Latif, a spokesman for Egypt's Interior Ministry. He said it only damaged the facility's walls and caused no casualties.

Ahmed Mahmoud, an engineering student living close by, said that the blast shook his building and caused a brief power outage. Mahmoud said that angry residents quickly blamed the Morsi's outlawed Muslim Brotherhood group for the violence and vowed to attack any Islamist rallies in their neighbourhood.

“People were saying they will carry arms and kill all Muslim Brothers who dare to pass from this place,” he said.

The interim government has blamed the Brotherhood for violence after the coup and has designated it as a terrorist organisation. The Brotherhood, which denounced violence in the 1970s, has denied any links to the terrorist attacks. However, the near-daily protests carried out by the group since the July coup often devolve into violence.

Morsi's supporters have vowed to continue marching, vowing to “break the coup.” The tense atmosphere created by the bombings, plus the chance that Islamist marches might encounter rival pro-military processions in the streets, increases the likelihood of violence.

At Tahrir Square on Saturday, the crowd also chanted: “The people want the execution of the Brotherhood.”

The Health Ministry said that a total of 15 people were killed Friday when Morsi's supporters armed with gasoline bombs and firearms loaded with birdshot clashed with security forces. The Interior Ministry said that 237 people were arrested during the protests.

In its most recent statements late Friday, Morsi's Freedom and Justice party, the political arm of the Brotherhood, renewed calls for Egyptians to rally in “peaceful revolutionary escalation” while condemned the “criminal bombings.” It said it held authorities responsible for failing to secure the country. - Sapa-AP