CAIRO - Egypt's public prosecutor on Friday ordered the detention for 15 days pending investigation of a former diplomat and other opposition figures who have criticized President Abdel Fattah al-Sisi, the state-run MENA news agency reported.Masoum Marzouk, a former ambassador, and two other prominent opposition activists were arrested on Thursday. Marzouk had recently called for a referendum on Sisi's rule, in rare public criticism of the former general.Marzouk, academic Yahya Kazaz, Raed Salama and four other people, whom MENA did not name, face charges of joining a terrorist group and receiving funds for terrorism, it said.Egyptian authorities have jailed thousands of Sisi opponents and critics in recent years, including alleged Islamist militants and secular rights activists. The government has said that its actions are directed at terrorists and saboteurs trying to undermine the state.The former general became president in 2014, a year after he led the ouster of Islamist president Mohamed Mursi after popular protests against his rule. Mursi was Egypt’s first freely elected head of state.The standard fifteen-day detention can be renewed by authorities. Rights groups have said this often happens indefinitely without trial to people accused of terrorism-related charges.