The military council that took power after veteran president Omar al-Bashir was ousted earlier this month meanwhile invited rally leaders to a new meeting.
Siddiq Farouk, one of those leading the protests, told reporters the demonstrators were "preparing for a general strike" across the country if the army rulers refuse to hand power to a civilian administration.
He also said that a "million-strong march" is planned for Thursday, confirming a call for a mammoth protest by the Sudanese Professionals Association, the group that launched protests against Bashir in December.
For the first time, Sudanese judges have said they will join a sit-in outside army headquarters "to support change and for an independent judiciary".
The demonstrations began in the central town of Atbara on 19 December against a decision by Bashir's government to triple bread prices.