The demonstrators began gathering in front of national security headquarters in the centre of the capital, but police and security agents quickly dispersed them, the witness said.
"Agents in civilian clothes beat with batons several young men trying to gather," he said. "They arrested several people and dragged them away." Some protesters briefly held up pictures of jailed relatives, Reuters reports.
Some 2,000 people have been detained since small protests began last month against austerity measures and the 23-year rule of President Omar Hassan al-Bashir, Amnesty International and Human Rights Watch said last week, citing Sudanese activists.
Sudan has had no mass protests like those in Egypt or Yemen but opposition to Bashir, in power since 1989, is increasing, acclerated by the country's economic problems.
In another sign of dissent, around 60 journalists protested against a security crackdown on independent newspapers, witnesses said. They shouted "freedom, freedom" in front of the Sudanese Union of Journalists.
Since the start of the year the authorities have closed several newspapers and confiscated dozens of entire editions before publication to stop critical coverage, editors say.
Sudan is suffering a severe economic crisis, resulting from the loss of much of its oil when South Sudan broke away and became independent a year ago.
Year on year inflation was 37.2 percent in June, double the rate of June 2011, adding to the hardships faced by millions of Sudanese after years of crises, ethnic conflicts and U.S. sanctions.
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