Protesters burnt tyres and branches in the streets and attempted to storm a government building before being rebuffed by security officials, witnesses said.
In Atabara, 300 kilometres northeast of the capital, riot police and plain-clothed operatives deployed tear gas against hundreds of protesters, a witness said.
Late Sunday evening, hundreds of people poured out of a football stadium after a match in Khartoum's twin city of Omdurman calling for "freedom, peace and justice", according to witnesses.
They were met by riot police who fired teargas at the crowds.
A wave of unrest has rocked Sudan since Wednesday after the government hiked the price of a loaf of bread from one Sudanese pound to three (from about two to six US cents).
Clashes with riot police killed at least eight people -- two in Atbara and six in Al-Qadarif to the southeast of the capital, according to official figures.
A higher death toll of 22 has been given by Sudan's main opposition leader Sadiq al-Mahdi, who blamed "armed repression" for the casualties.
'Cell of saboteurs'
There was a tense calm in the streets of Khartoum on Sunday as schools and universities were shuttered and riot police equipped with batons and tear gas guarded buildings.
"We were asked to leave this morning," said a university student from northern Khartoum.
Frustrated Sudanese queued outside bakeries in the city, where vendors were refusing to sell more than 20 loaves of bread per person.
"I have a big family and this bread is not enough for three daily meals," a local resident told AFP.
A bakery worker said a security guard standing nearby was not allowing the shop to sell any more.
Sudan's foreign ministry summoned Kuwait's ambassador to Khartoum Bassam Mohammed al-Qabandi after the Gulf state issued a travel warning and called on its citizens living there to leave.
Sudan's official news agency SUNA reported Sunday that authorities had arrested a "cell of saboteurs" that planned "acts of vandalism in the capital".
The official outlet said the "cell" includes members opposition groups, but did not elaborate.
Sadiq Youssef from the opposition coalition, National Consensus Forces alliance, had said earlier that 14 members of his group, including its president Farouk Abu Issa, were arrested as they left a meeting.
A group bringing together representatives of different professions called in a statement Sunday for a series of strikes over the price rise, starting with hospitals from Monday.
Sudan is mired in economic difficulties including an acute foreign currency shortage and soaring inflation.
Inflation is running at close to 70 percent and the Sudanese pound has plunged in value, while shortages have been reported for the past three weeks across several cities including Khartoum.
Demonstrations broke out in January over the rising cost of food, but they were soon brought under control with the arrest of opposition leaders and activists.