Thousands of protesters marched on the Sudanese army headquarters on Saturday, some of them reaching the complex for the first time since the deadly rallies against President Omar Al Bashir began in December. Footage posted online showed hundreds of protesters, mostly young people, marching toward the military headquarters in Khartoum, which is also the defence ministry. The mobilisation represents the most serious escalation of popular unrest against the Sudanese leader and the biggest challenge to his three-decade rule. Chanting "One Army, One People", the protesters rallied in the capital's streets, following a call by organisers to march to the army headquarters, located near the residence of Mr Al Bashir. Elsewhere, the large opposition Umma Party said security forces arrested four of its leaders ahead of planned marches in the province of Sennar, around 360 kilometres east of Khartoum. Protests have rocked the east African country since December, with angry crowds accusing Bashir's government of mismanaging the economy that has led to soaring food prices and regular shortages of fuel and foreign currency. On February 22, the veteran leader imposed a nationwide state of emergency to quell the protests after an initial crackdown failed to rein in protesters. Since emergency rule came into effect, the demonstrations have been largely confined to the capital and its twin city of Omdurman, but organisers had called for widespread rallies and a march on the army headquarters on Saturday. April 6 was chosen for the nationwide rallies as it was the day of a 1985 uprising that toppled the then regime of president Jaafar Nimeiri. The military removed Nimeiri after a popular uprising in 1985, before handing power over to an elected government, which in turn lost power to Mr Bashir in a coup supported by his Islamist hardliner allies. Before the protests began, security forces deployed in large numbers in key Khartoum squares and in Omdurman, across the Nile. In recent days, activists were circulating leaflets urging residents to participate in Saturday's march, according to residents. The protest movement was initially led by the Sudanese Professionals Association, but later several political parties including the main opposition National Umma Party threw their support behind it. The veteran leader has remained defiant however, and has introduced tough measures that have seen protesters, opposition leaders, activists and journalists arrested. Officials say 31 people have died in protest related violence so far, but Human Rights Watch has put the death toll at 51, including children and medics.