Crowds of Sudanese protesters chanted "freedom, freedom," as riot police fired tear gas on anti-government rallies in the capital and its twin city of Omdurman on Monday, witnesses said.
They said protesters took to the streets in two districts of Khartoum and in Omdurman, across the Nile.
Riot police swiftly moved in to disperse the protests, firing tear gas at one of the rallies in Khartoum and in the twin city, according to witnesses.
Even as police fired tear gas, protesters kept up the "freedom, peace, justice" rallying cry of an anti-government campaign that erupted in December, witnesses said.
Monday's demonstrations came after campaigners called for new rallies this week against the government of President Omar al-Bashir.
Deadly protests have rocked the east African country since December after cash-strapped Khartoum cut a vital subsidy on bread.
The protests quickly escalated into anti-government demonstrations across cities and towns, with protesters calling on President Bashir to step down.
Officials say 30 people have died in nationwide protests, but Human Rights Watch says at least 51 people have been killed.
President Bashir has remained defiant, addressing loyalists at several rallies across the country and seeking support from regional allies.
On Sunday, he held three rallies in the state of North Kordofan where he pledged to bolster rural growth by undertaking new infrastructure projects.
President Bashir and other senior Sudanese officials have repeatedly said the government can only be changed through elections.
The veteran leader, who came to power in an Islamist-backed coup in 1989, is considering running for a third presidential term in polls due next year.
The protest campaign led by the Sudanese Professionals Association is seen as the biggest challenge yet to President Bashir's three-decade-rule.