The trial of former Egyptian President Hosni Mubarak has resumed in Cairo amid renewed uproar between defence and prosecution lawyers. Some of the lawyers are reported to have withdrawn from the courtroom, saying it was too chaotic. Security was tightened after Monday's session descended into scuffles both inside and outside the Cairo courtroom. Mr Mubarak, 83, faces charges of ordering the shooting of protesters during the uprising that ousted him. 'Uncivilised' proceedings The fourth session of the trial opened with uproar in court, reports the BBC's Bethany Bell from Cairo. Anti-Mubarak protesters chanted "revenge, revenge," and called for the leader of Egypt's ruling military council, Field Marshal Tantawi, to give evidence in court. The judge told them that their conduct was uncivilised. Mr Mubarak was wheeled into court on a stretcher and joined his sons in the defendant's cage. The court is hearing evidence from senior police officers who were on duty during the uprisings in Egypt earlier this year. One officer was arrested for changing his evidence, a source inside the courtroom has told the BBC. The officer had originally told prosecution lawyers that he was given orders to use live ammunition against protesters, but changed his testimony in court. At the last hearing on Monday, none of the witnesses who testified implicated Mr Mubarak in the deaths of protesters during the revolution against his three-decade autocratic rule. Outside the courtroom, meanwhile, there have been small protests, despite the heightened security. "We are not tired... there is no alternative to the revolution," chanted the protesters in Arabic. Some 850 people were killed during the 18-day uprising in Cairo's Tahrir Square in January and February. The victims' families want to know what orders Mr Mubarak gave to his officials as police tried to stop the mass protests that resulted in the president's resignation on 11 February. The trial of Mr Mubarak and his sons Gamal and Alaa has been merged with that of former Interior Minister Habib al-Adly, who is also accused of ordering the killing of protesters. All the defendants deny the charges.