Supporters of opposition parties, and the junior partner in Sudan's ruling coalition, marched on Sudan's National Elections Commission - set up as an independent body to run the April poll - saying the organization was siding with the country's dominant party and introducing harsh restrictions.
The oil-producing state is preparing for its first fully multi-party presidential and legislative elections in 24 years, a ballot promised in the 2005 peace deal that ended Sudan's two- decade north-south civil war.
Sudan's dominant National Congress Party (NCP) and the former southern rebel Sudan People's Liberation Movement (SPLM) have already accused each other of fraud and intimidation during early campaigning and voter registration.
Protest leaders said new rules published by the National Elections Commission meant parties had to give 72 hours notice for rallies held inside their own party premises - and had to ask police for permission to meet outside.
Mubarak Al-Fadil, presidential candidate for the opposition Umma Party for Reform and Renewal, said he was also worried about reports the government was going to switch the printing of voting slips from abroad to presses inside Sudan - a move he said would lead to duplicate papers and ballot box stuffing.
"It seems the National Elections Commission is working very closely with the National Congress and is issuing circulars that infringe on the rights of political parties," said Yasir Arman, presidential candidate for the SPLM, which joined a troubled coalition government with the NCP after the 2005 accord.
Arman told Reuters he was boycotting state media even though his and all other parties were guaranteed equal 20-minute publicity slots on government television. "We have 20 minutes and the National Congress has 23 hours and 40 minutes," he said.
No one was immediately available to comment from the NCP, headed by Sudanese President Omar Hassan Al-Bashir.
Elections Commission member Mukhtar el Asam dismissed the accusation his organization was biased. "As far as I am concerned there is no substance to it. But the Commission is going to meet and give a full answer," he told Reuters.
Arman said there had been some progress in negotiations between the NCP and SPLM on other longstanding sticking points on the elections and other parts of the peace deal.
He said there had been a proposal to offer south Sudan 40 more seats in parliament to overcome southern accusations that a census undercounted the number of people in the region.
"We did not reach a final agreement. There are proposals to resolve it. ... But there is no time. We will need to recall parliament to approve them," Arman said.
SPLM officials told Reuters there were disagreements over who would get the 40 seats. The SPLM is keen to maintain a large enough majority to block constitutional changes.
Both sides remain at loggerheads on the position of parts of their shared border. And they have not named the members of a separate commission to run a referendum on southern secession, another contentious vote promised by the peace deal in January 2011.
