Omar al-Bashir, Sudan's president, has said anti-government protests against high prices is the work of "a few agitators".
More than a week of protests sparked by austerity measures have spread across the capital Khartoum and other cities, expanding beyond the core of student activists who hoped to turn public discontent into an "Arab Spring" movement.
Dismissing the demonstrations in a speech late on Sunday, Bashir said: "They said the economic measures were a chance for the Arab Spring, but we've already had the Arab Spring a number of times.
"When the Sudanese people revolt, they all come out. The people who are burning tyres are a few agitators."
Witnesses on Sunday evening reported groups of about 100 people demonstrating in two areas of Khartoum.
Security forces in the Red Sea town of Port Sudan beat people who were about to protest there, arresting some, witnesses also reported.
State security officers ordered one of Sudan's main opposition parties, Umma, not to hold any public forums at its headquarters, a party leader said.
"We are going to discuss this in our political bureau and decide what do," the party's general secretary, Ibrahim Al Amin, told AFP.
In El-Obeid, the capital of North Kordofan state, about 100 students from a local university called for the downfall of the regime, while other demonstrators gathered in the town's main market, witnesses said.
Police responded with tear gas and batons, they added, in what has become the standard reaction by security forces since demonstrations began on June 16 in Khartoum.
A Reuters reporter saw a heavy police presence in the area and rocks strewn on a street near the campus. The police were not immediately available to comment.
Bashir, an army officer who seized power on June 30, 1989, withstood earlier student-led protests by thousands of people objecting to high prices in 1994.
Though smaller in size, the current protests have occurred continuously for more than a week, with citizens blockading streets and throwing stones at police.
Lost oil revenues
The demonstrations follow the government's announcement last week of spending cuts to tackle an economic crisis worsened by the secession of oil-producing South Sudan a year ago.
Sudan has faced high food inflation and a weakening currency since South Sudan seceded, taking with it three quarters of the country's crude production - previously the main source of state revenues and foreign currency.
The measures include a cut in fuel subsidies.
Activists and opposition groups have been trying to use discontent over rising food prices and other economic woes to build a broader movement to end Bashir's 23-year rule.
But although spread across a wide variety of neighbourhoods, the demonstrations have so far lasted for only short periods and rarely exceeded more than a few hundred people at any one time
Plagued by inflation, and with a severe shortage of dollars to pay for imports. the country's poverty rate is 46.5 per cent, the UN says.
Sudan's demonstrations remain small compared with the mass uprisings that swept some Middle East countries last year as part of the Arab Spring protests.