Protesters against Egypt's military rulers remain camped in Cairo's Tahrir Square for a third day, after the deaths of at least 13 people in violence over the weekend.Security forces launched a major assault to clear the square on Sunday, but protesters returned within an hour.The unrest casts a shadow over elections due to start next week.It is the longest continuous protest since President Hosni Mubarak was ousted in February.Demonstrators say they fear Egypt's governing Supreme Council of the Armed Forces is trying to retain their grip on power.The council, led by Field Marshal Mohamed Tantawi, is charged with overseeing the country's transition to democracy after three decades of autocratic rule under Mr Mubarak.'Worst violence in months'Clashes were reported late into the night, with the injured being taken to makeshift clinics on the streets.Demonstrators on Sunday were seen throwing stones and petrol bombs at armoured personnel carriers and police. Security forces responded with tear gas and rubber bullets.Witnesses reported scenes of panic when hundreds of soldiers and police beat protesters on their heads as they chased them out of Tahrir Square.Protesters - some of them brandishing spent bullet casings - accuse security forces of also using live fire, a claim denied by police.Violence has also taken place in other cities including Alexandria, Suez and Aswan.A total of 11 people were reportedly killed on Sunday and two on Saturday, according to medical sources. Health officials say as many as 900 have been injured, including at least 40 security personnel.A statement from the cabinet said elections, due to begin in a week, would go ahead, and praised the "restraint" of interior ministry forces against protesters.The military council, in a statement read out on state television, said it "regretted" what was happening, AFP news agency reports.In recent weeks, protesters - mostly Islamists and young activists - have been holding demonstrations against a draft constitution that they say would allow the military to retain too much power after a new civilian government is elected.They have repeatedly tried to regain a foothold in Tahrir Square - the focal point of the protests against Mr Mubarak - but until this weekend they had always been removed quickly by the police.The latest violence is some of the worst in months between the Egyptian authorities and demonstrators.Parliamentary elections are scheduled to begin on 28 November. In all, the election process will take three months to complete.Earlier this month, the military council produced a draft document setting out principles for a new constitution.Under those guidelines, the military and its would be exempted from civilian oversight.This has angered protesters who fear the gains they have made during the uprising could yet slip away as the military tries to retain some grip on power.