BBC Online : Rival political groups in Egypt are to mark the third anniversary of the 2011 uprising which ended in the overthrow of President Hosni Mubarak. Both supporters of the military-backed government and the Muslim Brotherhood are set to take to the streets. The anniversary comes as tensions were heightened by a series of bombs in Cairo and clashes across the country which left at least 18 people dead. The government has said extra security measures are in place for Saturday. Egyptian Interior Minister Muhammad Ibrahim urged Egyptians not to be afraid to go to events marking the anniversary of the uprising. But shortly before 06:00 GMT on Saturday, a bomb was thrown at the wall of the police training academy in the Cairo suburb of Ain Shams, reportedly injuring one person. No-one has said they carried out the attack. Al-Qaeda-inspired militant group – Ansar Beit al-Maqdis (Champions of Jerusalem) – has said it was behind a deadly attack on the city’s police headquarters on Friday. Earlier Ibrahim warned Brotherhood supporters that any attempt to disrupt festivities would be dealt with firmly. The Brotherhood has held regular protests since the military, led by Gen Abdel Fattah al-Sisi, deposed Islamist President Mohammed Mursi last July. The BBC’s Yolande Knell, in Cairo, says that three years on from an uprising that raised hopes of political reform in the Arab world’s most populated country, rival demonstrations look set to show the deep political divisions. The Anti-Coup Alliance, led by Mursi’s Muslim Brotherhood, called in a statement for 18 days of protests beginning on Saturday, mirroring the 18 days of protests that led to Mubarak stepping down in 2011. The Brotherhood has been declared a “terrorist organisation” and accused by the interim government of being behind a string of violent attacks in recent months, which the Brotherhood denies. Mursi, Egypt’s first ever democratically elected president, was removed after mass protests. Many now expect Gen Sisi to run for president, putting a military strongman back in charge in Egypt, as was the case for the six decades leading up to 2011. On Friday six people were killed and some 100 others wounded in a series of blasts across Cairo, with the biggest blast outside the city’s police headquarters. That explosion killed four people and wounded at least 76, laving a huge crater in the street.