Egyptian authorities have detained a prosecution witness in the trial of former President Hosni Mubarak on charges of perjury after he allegedly changed his story concerning the use of live ammunition on protesters.

Officials said Wednesday that police chief Mohammed Abdel-Hakim denied any knowledge of security forces being equipped with live ammunition in late January as they tried to crack down on dissent. But prosecutors say Abdel-Hakim had previously stated that he knew live ammunition was used.

Mr. Mubarak arrived in the Cairo courtroom Wednesday on a stretcher, just as he has for previous sessions during which he lay on a bed inside the court's large metal cage. He has pleaded not guilty to charges that he ordered the killing of 850 protesters during the uprising that drove him from power in February. He is also facing charges of corruption and abuse of power.

At Monday's session, a top police official said he was not aware of any order to shoot live ammunition at protesters. Prosecution lawyers were visibly stunned by the testimony from General Hussein Moussa, which bolstered claims by Mr. Mubarak's backers that he did not orchestrate the violence.

Prosecutors accused Moussa of changing his story from the affidavit he had initially submitted to the court.

Mr. Mubarak is being tried along with former interior minister Habib al-Adly and six deputies.