At least four protesters were killed and many injured by gunfire in the Sudanese city of Omdurman on Thursday, opposition medics said, as hundreds of thousands took to the streets to pile pressure on the country's military rulers.

Organizers had called for a million-person march in cities across Sudan in response to the killing of young protesters in El-Obeid, southwest of Khartoum, earlier this week.

Sudan has been gripped by months of political turmoil that climaxed in the army overthrowing long-time leader Omar al-Bashir in April. Protesters have kept up their rallies since then, pressing the army to hand over to civilians, accusing the security services of violence and decrying the shortages and economic hardships that triggered the unrest in the first place.