An Egyptian court on Saturday overturned a mass death sentence against 37 supporters of ousted president Mohammed Morsi and ordered a retrial, a judicial source has said.The Court of Cassation, the country's highest appellate court, also overturned life sentences against 491 supporters of the ousted president, the source said.Last April, a court in the southern Minya province sentenced 37 Morsi supporters to death and 491 others to life in prison over killing a policeman in August of 2013 after security forces violently dispersed two pro-Morsi sit-ins in Cairo and Giza, killing hundreds of protesters in the process.The court had also acquitted 17 people in the same case.On Saturday, the Court of Cassation rejected an appeal presented by the prosecution against the acquittal verdicts.Egyptian authorities have launched a massive crackdown on supporters of Morsi and his embattled Muslim Brotherhood group, detaining thousands and killing hundreds.In December of 2013, the military-backed authorities declared the Brotherhood a "terrorist" group following a spate of attacks against security facilities. The authorities accuse the Brotherhood of condoning violence, a claim dismissed by the group, which says it is committed to peaceful activism.