(CNN) -- Voters in Sudan flocked to polling stations Sunday in the first multiparty elections in 24 years amid allegations of fraud and calls for a vote boycott.
There was confusion at some polling centers in southern Sudan as voters arrived to find their names were not on the list. Others complained about the complicated 12 levels of voting in the forms and lack of presidential pictures on lists in widely illiterate communities.
Up to 750 international and 18,000 domestic observers are assessing the election. One of them is former U.S. President Jimmy Carter, who said monitors had received "varying reports" of problems at polling stations around the country's capital.
"Most of the problems I saw this morning were logistical in nature and have already been corrected, at least around Khartoum," Carter told CNN. But he said some problems are to be expected "in a country that hasn't had an election in 23 years or so."
Voting is scheduled to continue through Tuesday, and Carter said he plans to visit Juba -- the capital of southern Sudan -- on Monday. The region's president, Salva Kiir, voted for what he said was the first time in his life and called the balloting a significant milestone for his country.
Voters were generally calm in the Juba area as they cast ballots and got a blue ink stain on their hands in exchange. Most stores were closed and security forces kept a watchful eye. But the Sudan People's Liberation Movement, which controls the south, said irregularities in the region were widespread Sunday.
SPLM spokesman Yein Matthew told CNN that in some cases, voters were presented with an additional list of candidates when polls opened and had to be presented with correct ballots. The group plans to present a list of incidents it has documented on Monday, Matthew said.
"There are so many, and we are still tracking them down," he said.
The SPLM recently withdrew its presidential candidate from the race against Sudanese President Omar al-Bashir, and opposition parties called for a boycott of this week's vote amid accusations of fraud against the ruling National Congress Party. Al-Bashir has denied the allegations, prompting some parties to withdraw partially or fully from the election.
The vote is taking place in accordance with a 2005 peace agreement that ended a decades-old conflict between Sudan's north and south. The peace agreement left them as separate territories, with a January 2011 referendum scheduled to decide whether they will reunite or sever their remaining ties.
"The holding of peaceful and credible elections is of paramount importance," U.N. chief Ban Ki-moon said in a statement Saturday. He said this week's vote "should contribute to the opening of political space in Sudan" ahead of 2011 vote.
But Susan Rice, the U.S. ambassador to the United Nations, said last week that the nation's election preparation was "quite disturbing." Rice said the United States is concerned about government restrictions on political freedom, freedom of speech and limitations on access to polling stations, particularly in the war-ravaged Darfur region in the west.
The decades of conflict between the north and the south left more than 2 million people dead and pitted Christian and Animist southerners against Muslim northerners. The peace deal, which mandated the election and the crucial 2011 referendum, was instrumental in ending the conflict.
"If these elections do not go according to plan and if a peaceful resolution isn't found, then I think it is very possible that we will see a return to conflict in Sudan," said Zach Vertin of the International Crisis group.
The other conflict in the western Darfur region, which gained more international media coverage, was a different war. It was between government militias and ethnic rebels. This conflict is regarded as genocide by the U.S. government and resulted in war crimes charges against al-Bashir, who took power in a 1989 military coup.
Al-Bashir's implementation of Islamic law created divisions between the north and south of the oil-rich nation. Analysts have said that as most of the oil is in the south, a split Sudan could negate sanctions to allow companies to prospect more easily in the South.
CNN's Ben Brumfield contributed to this report.