A key challenger to Sudanese President Omar al-Beshir in next month's elections accused the government on Thursday of plotting to rig the outcome.

Yasir Arman, who is standing against Beshir for the former southern rebel Sudan People's Liberation Movement, accused the electoral commission of restricting the campaigning of opposition parties and of paving the way for fraud by the government.

"The national elections commission recently put in place measures in line with the security and the police to restrict the freedoms of political parties. This is unacceptable," Arman said.

Around 200 to 300 opposition supporters demonstrated on Thursday in front the office of the elections commission, as opposition leaders presented the commission with a report outlining examples of lack of fairness.

Arman said voting cards were being printed by the government press.

"This is going to give the (ruling) National Congress the chance to rig the elections," he said.

Opposition parties have also criticised a security law that forces them to obtain police authorisation prior to any political meeting "even inside the premises of the political forces."

"That is new and that does not come from the security it comes from the national election commission," Arman said.

The general elections scheduled for April, will be the first multi-party polls since 1986.

Arman and Umma party chief Sadeq al-Mahdi are the two main challengers of Beshir, who is wanted by the International Criminal Court for alleged crimes against humanity in the western region of Darfur.

Mahdi, a former prime minister who was ousted in a 1989 coup by Beshir, cancelled a radio broadcast on Wednesday after national radio asked him to alter it.

Under electoral commission rules, the candidates are allowed an equal amount of airtime on national radio and television.

Arman, a secular Muslim from north Sudan who was chosen as the presidential candidate of the southern ex-rebel group the Sudan People's Liberation Movement (SPLM), has already stated his intention to boycott state media during his election campaign.

The electoral commission's rules on the candidates' use of air time do not apply to private television and radio stations. - Sapa-AFP