Ballot boxes are sent to state capitals, from where they go to Khartoum
Sudan's poll results, due on Thursday, will be delayed - and a full picture is unlikely to emerge until next week - says the National Elections Commission.
The BBC's James Copnall in Khartoum says the delay is because counting is taking longer than anticipated and other logistical problems.
Results so far, mainly from the north of the country, show President Omar al-Bashir's party has a strong lead.
The polls were among the most complex elections ever held, the UN says.
They were the first multi-party presidential, parliamentary and regional elections in Africa's largest country since 1986.
Africa's biggest country
Deeply divided along religious and ethnic lines
11 April elections intended to be first multi-party national poll for 24 years
Continuing conflict in Darfur
President Bashir wanted for war crimes in Darfur
South Sudan rebuilding after 21 years of civil war
South Sudan could secede in 2011
Large oil fields near north-south border
Two of President Bashir's main challengers have withdrawn from the poll, alleging fraud.
On Tuesday, a YouTube video was circulated which allegedly showed election officials stuffing ballot boxes, although the elections commission and Mr Bashir's National Congress Party (NCP) have dismissed the film as a fake.
The BBC's correspondent says the election results are being released in little spurts.
Most of the results so far have been from the north, with fewer from Darfur and particularly the south.
Our correspondent says not all of the ballot boxes have reached state capitals, from where they go to Khartoum.
Meanwhile, the Paris-based Sudanese newspaper, Sudan Tribune, reports that two lorries transporting cast ballot papers in the southern state of Western Equatoria had been seized and burned by bandits.
These reports have not been confirmed.
In results released so far, the NCP has all but swept the board, sometimes winning by more than 90% of the vote.
The Sudan People's Liberation Movement (SPLM) are expected to do equally well in the semi-autonomous south of the country, where the former rebel group are already in power.
But presidential candidate for the Popular Congress Party, Abdallah Deng Nhial, told the BBC this week his sizeable party had failed to win a single seat.
Both local and international observers have alleged the elections have been marred by fraud.
The big question now is not who wins, but how the Sudanese and the rest of the world react to the results, our correspondent says.