KHARTOUM - Southern former rebels called for polling to be extended in Sudan's first multi-party vote in two decades on Sunday as the electoral comission acknowledged "mistakes" in distributing ballot papers.

The Sudan People's Liberation Movement (SPLM) said it had asked for polling to be extended from three to seven days after what it described as a "wasted" first day marred by procedural problems across Africa's largest country.

"There have been a lot of irregularities that we have noticed," said Samson Kwaje, campaign manager for SPLM leader Salva Kiir.

"Today was a wasted day. We are seriously sending a protest to the NEC," the National Election Commission, he added.

Kwaje said the irregularities included polling stations opening late, wrong ballot boxes in the wrong places and ballot boxes going missing.

The NEC acknowledged there had been "mistakes" in distributing ballot papers in some areas but made no comment on the possibility of prolonging the election.

It said the process was going well "except some technical mistakes about the distribution of ballots in 26 centres in Khartoum state."

"In White Nile state, there has been a problem matching the symbols with the candidates," it added.

The polling process had always threatened to be difficult with an electorate with a high level of illiteracy contending with multiple ballots for simultaneous presidential, parliamentary, state and southern regional elections.

In the southern regional capital Juba, voters said they struggled with no fewer than 12 ballot papers.

But the complaints compounded question marks about the credibility of an election from which key candidates had already withdrawn ahead of polling day citing fears of fraud.

The SPLM pulled out its presidential candidate Yasser Arman, while former prime minister Sadiq al-Mahdi of the northern opposition Umma party also withdrew.

The opposition parties accused the National Congress Party of vetern President Omar al-Beshir, who seized power in an Islamist-backed coup in 1989, of plotting to fake an election victory, particularly after the contract for ballot papers went to a state-owned printer.

Beshir, who shouted "Allahu Akbar" (God is greater) after casting his ballot at the St. Francis school in central Khartoum, has promised an "exemplary" election.

In Juba, the southern former rebel leader said he was voting for the first time in his life and hoped the elections would lead to the "formation of a democratic process in south Sudan."

Kiir is standing for election as president of the autonomous government in south Sudan that will lead the region to a promised referendum on independence next January.

The two votes are central planks of a 2005 peace deal between the SPLM and Beshir's government that ended two decades of civil war between the mainly Christian and animist south, and the mainly Muslim north.

In the western region of Darfur, where civil war between the Khartoum government and ethnic minority rebels has been raging since 2003, international peacekeepers reported no violence.

"So far the situation is calm," one peacekeeper with the joint UN-African Union mission told AFP.

European Union monitors had pulled out of Darfur before polling day citing concerns about security in the region where an estimated 2.7 million people have fled their homes.

In March 2009, Beshir became the first sitting head of state to be indicted by the International Criminal Court. The court issued a warrant for his arrest on charges of war crimes and crimes against humanity in Darfur.

Darfur rebel groups which control parts of the vast region have firmly rejected this month's elections, but have so far not stated any intention to derail the process.

While there may be little doubt over the outcome of the presidential race after the withdrawal of Beshir's most credible challengers, the elections for parliamentary and state representatives are still fiercely competitive in many parts of the country.

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