Sudanese security forces have opened fire on demonstrators, killing at least four, as thousands marched through the streets of the capital in continuing protests over fuel price hikes.About 3,000 people, angered by a police crackdown on previous protests, also demonstrated after Friday prayers in Khartoum's twin city Omdurman, across the Nile, shouting "Freedom! Freedom!" and "The people want the fall of the regime!". Clashes also took place in the city of Wad Medani.Al Jazeera's Caroline Malone reports.