Sudanese riot police and security agents surrounded organisers of a protest against alleged election fraud on Sunday, witnesses said, in the latest sign of a clampdown following uprisings across the Arab world.
Sudan has regularly shut down demonstrations in the past, but the response of its security agencies has taken on an extra urgency since mass protests in neighbouring Libya and Egypt.
Police, armed with batons and teargas have quickly broken up dozens of small anti-government protests in north Sudan this year, many of them against rising prices and some calling for regime change. Dozens of activists are in jail.
More than 100 police and plain-clothed agents staked out the offices of the former southern rebel Sudan People's Liberation Movement (SPLM), which planned the protest in Khartoum, Reuters witnesses said.
Security officers also prevented people from approaching the nearby offices of Sudan's National Elections Commission to hand in a petition about irregularities in registering voters in the oil-producing state of Southern Kordofan, organisers said.
They turned the commission into a security garrison, senior SPLM official Yasir Arman told Reuters.
Arman said security officers stopped his supporters in streets around the area. He had finally got into the commission by a back route with representatives of nine opposition parties.
It is a violation of the constitution and it is a violation of human rights. It is what is happening in Egypt, Tunis and Libya, he added, referring to the security presence.
The SPLM has accused Sudan's dominant National Congress Party (NCP) of rigging the voter registration, adding more than 38,000 names to areas of Southern Kordofan it says are controlled by security services.
Senior NCP official Rabie Abdelati dismissed the accusation as political rumours and said peaceful demonstrations were allowed, as long as protesters applied for a licence.
Sudan's president and NCP head, Omar Hassan al-Bashir, held on to power in elections in April 2010. Gubernatorial and state assembly elections in Southern Kordofan were delayed until May this year after disagreements over an earlier census.
The SPLM signed a peace deal with the north in 2005 and joined a coalition government with the NCP. The SPLM has said it will form a separate opposition group in the north after the south's expected secession in July.