Video posted on YouTube claiming to show polling officers stuffing ballot boxes with fraudulent voting slips
Sudanese opposition activists have said a video which apparently shows election officials stuffing ballot boxes proves their claims of poll rigging.
The clip, which has not been independently verified, has been posted on the internet and is being circulated by a coalition of campaign groups.
The National Elections Commission (NEC), however, dismissed it as a fake and is not even investigating it.
The elections were held under a deal to end a 21-year north-south civil war.
The presidential, parliamentary and regional polls were the first multi-party elections since 1986.
Results are expected this week, after the elections were extended by two days due to organisational problems.
Several opposition parties withdrew in the north, saying President Omar al-Bashir's National Congress Party was trying to rig the voting.
On Tuesday, the US said the elections were plagued by "serious irregularities" and more should have been done to prevent them.
An election monitoring group earlier told the BBC it suspected that one of its observers had been kidnapped and beaten in the semi-autonomous south.
The video, which was posted on the video sharing website YouTube and which is impossible to verify independently, was apparently filmed in the eastern Red Sea state.
It shows people wearing the distinctive orange bibs of election officials and traditional white gowns seemingly stuffing ballot boxes.
It has been circulated by the Sudan Democracy First Group - a coalition of trade unions and activists.
"This video is proving everything we said that the elections are rigged and they rigged the boxes," opposition Communist Party official Siddig Youssef told the Reuters news agency.
The presidential candidate of the opposition Popular Congress Party (PCP), Abdallah Deng Nhial, described the video as "scandalous".
"It's forgery, of course, done by the ruling party. They changed the boxes with already filled boxes. They brought them by cars from outside."
However, National Elections Commission member al-Hadi Mohamed Ahmed told reporters that no complaint had been received.
"We will not investigate anything that appears on the internet."
A senior advisor to President Bashir, Ghazi Saleheddine, also cast doubt on the video.
"It could be made up, I have seen people wearing orange tunics which could be anywhere and boxes can be also found so it's just a video, they only have their word for it," he said.
At the weekend, the EU and the Carter Center, led by former US President Jimmy Carter, said the polls had fallen short of international standards.
However, both concluded the 11-15 April vote was a significant step towards democracy.
President Bashir is expected to be re-elected, after his two main challengers withdrew, alleging fraud.
The former rebel Sudan People's Liberation Movement (SPLM) is equally expected to retain power in the south.
The complicated ballot in Africa's largest country was beset by problems and heavily criticised by the Sudanese opposition and local observers.
Observers said the ruling parties in both the south and the north also used their huge advantage in resources to influence the vote.
The EU's team was withdrawn from Darfur, where low-level civil war continues, because of fears about safety and whether the monitors could observe freely.