Sudanese police have fired tear gas at people protesting at recent austerity measures in the capital Khartoum, witnesses say.
Some of the demonstrators reportedly carried anti-government placards as President Omar al-Bashir prepares to mark 23 years in power, on Saturday.
This is the latest in two weeks of protests, initially led by students.
The government is trying to cut spending after losing much of its oil revenue when South Sudan seceded.
Following recent subsidy cuts, prices have risen - especially for food and fuel - and inflation has soared in the year since South Sudan's independence.
'Licking your elbow'
Several hundred protesters took to the streets after Friday prayers in Khartoum, witnesses in at least two neighbourhoods of the city reported.
At one of the city's largest mosques some were chanting "freedom, peace, justice and revolution is the choice of the people", Reuters news agency reporter Yara Bayoumy told the BBC's Focus on Africa programme.
As police fired tear gas some demonstrators responded by hurling stones, she said.
A presidential aide once said that getting rid of President Bashir was as impossible as "licking your elbow".
The phrase has now become the defiant slogan of opposition activists.
The internet is full of messages about breaking the fear barrier and calling for more demonstrations on Saturday.
But correspondents say internet usage in Sudan is still quite low.
According to the AFP news agency, armed security agents raided their Khartoum bureau on Friday and arrested a part-time correspondent who had taken pictures of a protest.
There has been strong international condemnation of the arrest and treatment of some activists.
The BBC's Africa correspondent Andrew Harding says comparisons with the Arab Spring may be premature.
But the combination of economic gloom, and a series of internal armed rebellions, is pushing Sudan into unpredictable territory, he says.