Authorities at Cairo airport on Sunday blocked entry for Sudan's main opposition leader Sadiq al-Mahdi, who has been living in exile in Egypt, his party said.The Umma Party said security stopped former prime minister Mahdi from entering the country as he returned to Cairo after participating at an opposition conference in Berlin. Mahdi, who has been based in Cairo, then flew to London with his daughter Mariam and an aide, his party said. A fixture of Sudanese politics since the 1960s, Mahdi was prime minister from 1966 to 1967 and again from 1986 to 1989.His government was the last one to be democratically elected in Sudan, before it was toppled by a 1989 coup launched by current President Omar al-Bashir.He briefly returned to Khartoum in January 2017 after more than two years of exile in Cairo, but had since returned to the Egyptian capital. In April Sudanese media reported that the state security prosecutor had charged Mahdi with collaborating to overthrow Bashir after a meeting with rebel leaders in Paris.Since the coup that ousted Mahdi's civilian government, the ex-premier's Umma Party has acted as Sudan's main opposition group and has regularly campaigned against the policies of Bashir's government.The Umma party said Egyptian officials had asked Mahdi not to participate in the Berlin meeting.There was no immediate response from the authorities in Cairo.* Sign up to News24's top Africa news in your inbox: SUBSCRIBE TO THE HELLO AFRICA NEWSLETTERFOLLOW News24 Africa on Twitter and Facebook.