Sudanese security forces were deploying in large numbers on Sunday in Khartoum and the capital's twin city in anticipation of fresh protests calling on longtime ruler Omar al-Bashir to step down, according to activists and video clips circulating online.The videos show hundreds of security forces in all-terrain vehicles in Khartoum and heading to nearby Omdurman, a traditional hotbed of dissent that saw hours of pitched battles between police and protesters last week.Sunday's anticipated protests come amid a series of strikes, already underway or planned for this week, by professional unions, including doctors, teachers, lawyers and pharmacists. Demonstrations are also expected in other cities on Sunday.Al-Bashir, who seized power in a military coup nearly 30 years ago, insists there will be no change of leadership except through the ballot box. Already one of the longest serving leaders in the region, he is expected to run for a new term in office in elections next year.An Islamist wanted for genocide by the International Criminal Court, al-Bashir has repeatedly warned that the protests could plunge Sudan into the kind of chaos convulsing other countries in the region.* Sign up to News24's top Africa news in your inbox: SUBSCRIBE TO THE HELLO AFRICA NEWSLETTERFOLLOW News24 Africa on Twitter and Facebook. The protests erupted December 19, initially over price hikes and shortages, but soon shifted to calling on the president to step down. Rights groups last week said at least 40 people have been killed in the protests, while the government acknowledged 24 deaths.Activists say three protesters, including a doctor and a child, were killed in Thursday's clashes, but a police spokesman said the next day that only two were killed. He denied that security forces used live ammunition. Five doctors have died in the protests, according to the independent physicians union.Although participants have so far been in the hundreds or low thousands, the continuing protests and strikes pose a challenge to al-Bashir's rule. The absence of explicit support by Sudan's Arab allies has made al-Bashir's position even more tenuous.However, the protests show a lack of clear leadership and their continuation could invite another military takeover. Union leaders say they want a transitional government of technocrats followed by free elections.Al-Bashir's position was further weakened when a senior cleric revealed that he and fellow clerics have implicitly suggested to the Sudanese leader in a meeting that he step down, arguing that Islam takes precedence over individuals.In a surprise disclosure in a Friday sermon, Sheikh Abdul-Hay Youssef said the clerics presented al-Bashir with a list of demands, including an end to corruption, bringing to justice anyone found responsible for the latest economic crisis and an end to the killing of protesters, something that the Sudanese leader has sought to justify on religious grounds.