Police and protesters have clashed for a second day in the Egyptian capital, Cairo, after the demonstrators reoccupied Tahrir Square.Officers fired volleys of tear gas as they fought stone-throwing activists advancing on the interior ministry.Two people were killed on Saturday in Cairo and Alexandria.The demonstrators say they fear Egypt's military rulers are trying to retain their grip on power, in spite of elections due in just over a week.The violence in Cairo began early on Saturday when riot police tried to remove protesters who had spent the night in Tahrir Square following protests on Friday.After clashes that lasted for much of the day and well into the following night, the police eventually pulled back into the surrounding streets, leaving thousands of protesters in control of the square.A makeshift camp has sprung up in the square - exactly like the one that appeared during the protests in February that forced President Hosni Mubarak from power.A temporary field hospital has been set up to treat injured protesters.Tahrir Square - which is usually one of Cairo's busiest traffic thoroughfares - is now closed to vehicles at the start of the working week and most nearby businesses are shut.Renewed violenceProtesters - 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 gain a foothold in Tahrir Square again, but up to now they have always been removed quickly by the police."The violence yesterday showed us that Mubarak is still in power," one protester, Ahmed Hani, told the AP news agency.He said the leader of Egypt's military government, Field Marshal Hussein Tantawi, should resign."We have a single demand: The marshal must step down and be replaced by a civilian council," he said.Saturday's violence was some of the worst in months between Egyptian police and demonstrators.Health officials say as many as 900 people were injured, including at least 40 security personnel.The demonstrators in Cairo set a government building and a police vehicle on fire and threw stones and petrol bombs at riot police.The number of protesters grew during the day, reportedly after a call went out on social media for people to join the demonstration following the police assault.Government warningPrime Minister Essam Sharaf called on the protesters to clear the square."What is happening in Tahrir is very dangerous and threatens the course of the nation and the revolution," a statement from the cabinet said.One of the demonstrators, Ali Abdel Aziz, said security forces beat up protesters to break up the sit-in on Saturday."They beat us harshly, they didn't care for either men or women," the 32-year-old accountancy professor told the AFP news agency.Parliamentary elections are due to begin on 28 November and take three months.Earlier in November, Egypt's military rulers produced a draft document setting out principles for a new constitution.Under those guidelines, the military would be exempted from civilian oversight, as would its budget.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.