The withdrawal of so many parties is a serious blow to the poll's credibility
Sudan's President Omar al-Bashir has insisted that the forthcoming elections will be "free and fair".
In a speech on the campaign trail, Mr Bashir said the poll would be clean as it was "a religious duty".
Sudan's elections were intended to be the first multi-party polls since 1986, and pave the way for real democracy in Africa's largest country.
But several major opposition parties have announced they will boycott the polls, in protest at alleged rigging.
Only minor opposition parties are now taking part in northern areas, and the main party of South Sudan, the SPLM, is only contesting in the south.
A serious blow
The electoral process has been heavily criticised by international and local rights groups, as well as by parties opposed to the president.
The BBC's James Copnall in Khartoum says Mr Bashir dismissed these criticisms, much as he swept aside the opposition's demand that the polls, scheduled for Sunday to Tuesday, be postponed.
The president has been very active during the campaign, announcing new development projects in every town he has visited, our correspondent says.
"The elections will be fair and free and clean and exemplary," Mr Bashir told a gathering in northern Sudan.
"They will be clean because elections are a religious duty," he said in a speech broadcast on state television.
In contrast, the opposition parties have spent more time recently debating whether to run in the elections than in trying to convince the voters, our correspondent says.
Both of President Bashir's main rivals, Sadiq al-Mahdi of Umma and Yassir Arman of the former southern rebels, the SPLM, are boycotting the presidential election.
President Bashir's National Congress Party will perhaps face more of a challenge in the parliamentary and state elections.
But our reporter says the withdrawal of so many parties has dealt a serious blow to the credibility of these elections.
Some Sudanese activists believe the international community is pushing for these polls to take place, even if they are not fair, as the real priority is next January's referendum on possible southern independence.
The north-south civil war ended in 2005, with a deal for the SPLM to share power with Mr Bashir's party at the national level, while enjoying considerable autonomy in the south.
Some 1.5 million people died in the conflict between the mainly Muslim north and the south, where most people are Christian or follow traditional beliefs.