CAIRO - Egypt's military ruler and Hosni Mubarak's former confidant is testifying in the trial of the ousted president charged with complicity in the killings of protesters during the crackdown on Egypt's uprising. Field Marshal Hussein Tantawi arrived on Saturday morning to the heavily guarded courtroom to give his testimony under a complete media blackout. Such high-profile testimonies are rare in Egypt. Tantawi, who was defense minister for two decades, could provide critical insight into Mubarak's alleged role in the deaths of nearly 840 protesters as well as insider revelations about the former regime's final days. Dozens of activists and families of slain protesters are rallying outside the Cairo court, chanting: "Tantawi, tell the truth, is Mubarak a killer or not."