Sudan's main opposition parties have withdrawn from presidential elections, a senior member of one of the groups said on Thursday.

"On the level of the candidates of the Presidency of the Republic, most of them (Sudan's opposition groups) decided to withdraw," said Mohamed Zaki, head of office for Sadeq al-Mahdi, leader of the opposition Umma party.

Zaki said only five independents and representatives of smaller parties were still in the race against President Omar Hassan al-Bashir.

Zaki said there was still a chance the main opposition candidates would review their decision if the government agreed to an overhaul of the country's National Elections Commission, and responded to their complaints of widespread fraud.

Thursday's announcement came a day after south Sudan's leading party, the former rebel Sudan People's Liberation Movement (SPLM), withdrew its candidate Yasir Arman from the presidential poll, protesting against what they say electoral irregularities.

The SPLM and opposition groups have demanded an investigation into how a government company won a tender to print voting papers and have made numerous complaints of fraud during voter registration and other preparations.

Sudan's Communist Party told Reuters it had decided to boycott all levels of the April elections. The Popular Congress Party announced its presidential candidate Abdullah Deng Nhial would stay in the race.

No one was immediately available for comment from Sudan's major opposition Democratic Unionist Party (DUP) to confirm it had withdrawn its candidate alongside Umma.

The SPLM on Thursday said it would also boycott all voting in Darfur, the scene of a seven-year conflict.

Reuters