Sudan's largest opposition groups are calling for a fresh wave of protests against longtime autocrat President Omar al-Bashir, with a series of demonstrations starting this week.In a joint statement on Thursday, four groups called for nationwide protests on Friday and a march on the president's Nile-side palace on Sunday. They also called for a march on parliament in Khartoum's twin city of Omdurman to present a petition demanding that the 74-year-old general-turned-president step down.The demands are likely to turn up pressure on al-Bashir to find a way out of the crisis. Sudan has seen two weeks of violent street protests, with dozens killed. The protests were initially sparked by worsening economic conditions.Sudan's communist party and rights groups say up to 40 have died so far in the protests.* Sign up to News24's top Africa news in your inbox: SUBSCRIBE TO THE HELLO AFRICA NEWSLETTERFOLLOW News24 Africa on Twitter and Facebook.