The vote was guaranteed in a 2005 peace deal signed between the northern National Congress Party (NCP) and the former southern rebel Sudan People's Liberation Movement (SPLM) which ended more than two decades of civil war fought over religion, ethnicity, oil and ideology.
With elections three months away, relations between the former foes remain tense and election officials face a raft of logistical challenges.
Both the SPLM and the NCP have accused each other of fixing November and December's voter registration process. Opposition groups have also complained about fraud and intimidation at registration centres.
The U.S. envoy to Sudan, Scott Gration, this week said he was concerned about "ongoing restrictions to the freedom of speech, of assembly and of the press" in the run up to the campaign.
Newspaper censorship has been lifted but police used tear gas to break up two opposition rallies in December.
Analysts have warned there is a risk of a return to conflict if any of the main players in the north or south feel cheated by the poll's final outcome.
There has been a spike in tribal violence in south Sudan, with aid groups estimating at least 2,500 have been killed since the beginning of 2009.
Elections officials are worried the violence could make campaigning and voting next to impossible in affected areas.
The SPLM reject a 2008 census, which is set to be used to define electoral constituencies for the vote.
They say the NCP over-counted northerners and undercounted southerners, affecting the number of parliamentary seats for which the south would be eligible.
There are reports both sides may be discussing a way round the impasse, possibly by guaranteeing the south a larger proportion of seats, but no deal has been announced.
Some rebels in the festering conflict have opposed the poll and threatened to treat elections officials like enemy soldiers.
With millions of people driven from their homes to makeshift camps, the nomination of candidates and identification of voters will be thorny.
Ongoing clashes and violent banditry in the lawless region will make voting and observing difficult.
Officials have to organise the complex poll in Africa's biggest country, large parts of it underdeveloped and remote.
Few voters have any direct experience of taking part in elections. Around 39 percent of the population are thought to be illiterate and may struggle with complex elections documents.
Officials have little room to manoeuvre if they need to delay the poll to make up for delays in preparations. If they miss April, they will have to wait until the end of the south's rainy season in November.
That would push the elections uncomfortably close to a highly-charged referendum on southern independence, scheduled for January 2011.
(Reporting by Andrew Heavens and Opheera McDoom; Editing by Elizabeth Fullerton)
- Reuters - Thomson Reuters Foundation
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