It said the decision was in protest at what it called unfair electoral rules and fraud in the electoral process.
Activists cautioned on Saturday that the credibility of the election had been undermined by the allegations, which they said have been ignored by al-Bashir's ruling NCP.
"Violations of human rights - particularly restrictions on freedom of assembly and freedom of the press - are threatening prospects for a free, fair and credible vote across Sudan," Georgette Gagnon, Africa director of Human Rights Watch, said.
"Sudanese authorities are clearly failing to uphold international standards."
Gagnon is one of a group of international activists who gave warning of renewed violence in the country on the eve of the vote.
Hopes have dimmed that the elections will be a watershed moment as Sudan attempts to put decades of conflict behind it.
Last-minute boycotts have revealed the fragility of the 2005 peace deal that ended the country's long North-South civil war.
The NCP has ruled Sudan in a coalition with the SPLM since the signing of the peace agreement.
Part of that deal was an agreement that Southern Sudan could hold a referendum on independence and many in the south see these elections as step towards that goal, Hoda Abdel-Hamid, Al Jazeera's correspondent, reported from the region's main city, Juba.
"The people here are looking at these elections as a stepping stone to reach the referendum.
"It's a very important vote for the people in south Sudan, certainly more important than the vote that will happen in days to come."
Analysts said on Saturday that renewed violence over the outcome of the coming election is a distinct possibility.
"If elections are to take place in such compromised environment, conflicts about the legitimacy of the results might spark violence," the African Centre for Justice and Peace Studies said in a statement.
EU team withdraws
The European Union has pulled its election observation team out of Darfur, the conflict-torn western region, in the run-up to the vote.
The UN estimates that 300,000 people have died in the humanitarian crisis in Darfur, which has been described as genocide by Western powers and has formed the basis for an International Criminal Court indictment for al-Bashir.
The Obama administration said on Friday that it would consider supporting a brief delay to the elections to allow opposition concerns to be addressed. But the Sudanese election commission has said there is no prospect of the vote being delayed.
Meanwhile, a group of parliamentarians from 12 other countries called for the elections to be closely monitored for irregularities.
"We are deeply concerned about the inadequate level of preparation for the upcoming elections in Sudan and the threat of violence and intimidation to civil society," they wrote in an open letter.
"The building blocks of a free and fair election ... are not in place," the legislators said.
They belong to Australia, Afghanistan, Bahrain, Britain, Egypt, France, Germany, Italy, Kuwait, Mauritania, Morocco and Yemen.