Nigerian opposition parties threatened to boycott this Saturday's presidential election, adding to the turmoil that has surrounded one of the most closely watched elections here since independence.
Tuesday's announcement followed a Supreme Court ruling allowing the vice president to stand and came days after more than 20 people were killed in regional elections heavily criticized by the international community.
A group of 18 opposition parties said the results of last Saturday's vote should be voided, the national election commission should be disbanded and further elections should be postponed until "transparency and fairness" can be guaranteed.
If these conditions were not fulfilled, the parties "shall consider not participating in the election," they said in a statement.
The statement was read at the end of a meeting in the presence of two leading opposition candidates in the presidential election, Vice President Atiku Abubakar and General Muhamadu Buhari.
Earlier the election commission ruled that Abubakar could stand in the vote, reversing an earlier move to exclude him after the Supreme Court decided on Monday that the commission did not have the right to disqualify candidates.
Abubakar, excluded from the initial list of approved candidates following corruption allegations, is one of two opposition figures expected to pose a serious challenge to the ruling party's Umaru Yar'Adua in this Saturday's poll.
The other is former military ruler Muhamadu Buhari who is contesting under the banner of the All Nigeria Peoples Party (ANPP).
"The Commission assures Nigerians that the VP will be on the ballot," Independent National Electoral Commission (INEC) spokesman Philip Umeadi told a news conference.
The presidential vote, which will also determine the head of the legislative assembly, is expected to usher in Nigeria's first civilian-to civilian handover since the country gained independence from Britain in 1960.
But the regional elections, marked by the deaths of between 20 and 50 people, were slammed as fraudulent by the EU, the US and rights watchdogs such as Human Rights Watch and Amnesty International.
"In Nigeria, the United States takes seriously reports of voting irregularities and election-related violence during this past weekend's state and local elections," US presidential spokeswoman Dana Perino said.
"We would urge that officials address any problems in order to ensure that Saturday's presidential election, that those polls are free and fair and conducted in an atmosphere free of violence," she said.
EU observers who monitored the state governorship elections also expressed "serious concerns" about the vote.
The EU observer mission criticized the late opening of polling stations, under-age voting, failure to guarantee the secrecy of the ballot and a shortage of electoral officials.
There were "incidents of hijacking of ballot boxes by groups of thugs" as well as irregularities that led to significant discrepancies between results at polling stations and ballot collation at the local government level, they said.
The statement urged the electoral commission to ensure significant improvements before the presidential and legislative elections are carried out.
"Preparations should have been of a higher standard and must be substantially improved in the coming days to deliver a credible and secure process at the federal elections," the EU statement said.