LAGOS, Nigeria -- The top opposition candidate called Sunday for Nigeria's presidential vote to be annulled, branding the election as the worst ever conducted in Africa's most-populous nation and top oil producer.
While the government acknowledged there were widespread problems with Saturday's vote, it defended the election as free and fair. "The election has been largely successful: We've broken the jinx," said Electoral Commission Chairman Maurice Iwu.
Associated Press reporters, however, witnessed ballot-paper shortages in opposition strongholds and open rigging favoring the ruling party of outgoing President Olusegun Obasanjo. Local media reported intimidation by thugs with knives and guns.
Vice President Atiku Abubakar, who fell out with Obasanjo last year and ran as an opposition candidate, said no free and fair election could be arranged by the current electoral commission, which he accuses of partisanship toward the ruling party and Obasanjo.
"I have already rejected the election. They have no alternative than to cancel the election all together," he said. "What we have seen clearly prove our fears that it is the worst election that we ever seen."
The Transition Monitoring Group, an independent election monitoring group claiming 50,000 Nigerian observers, also called for the election to be annulled, saying voting hadn't been held in many of the country's 36 states and had started very late in many others.
"That's why we're calling for the cancellation of the entire exercise," said Innocent Chukwuma, the chairman of the body.
The other main opposition party of Gen. Muhammadu Buhari didn't immediately say it rejected the vote, but described it in disparaging terms. "Some voting has taken place, but there was no election," said Abba Kyari, a spokesman for the party.
International election observers have not yet commented on the conduct of the vote. Germany, which holds the rotating European Union presidency this month, said in a statement, however, that it was worried about the reports of irregularities and use of violence in some regions.
"These incidents have given rise to concerns that not all Nigerians entitled to vote really were able to do so freely and without fear," the statement said. Germany added it would carefully examine the final report from EU election observers, calling on the Nigerian government "to ensure that there are no doubts abut the credibility of the election results."
Obasanjo on Saturday hailed the vote as a major step forward for democracy in Nigeria. Iwu rejected the opposition's challenges to the vote's credibility.
"Anyone who says (the vote) was not free and fair wants to mess this country up," he said. "There's no question of legitimacy."
The presidential winner must gain the most votes nationwide and at least a quarter of ballots cast in 24 of Nigeria's 36 states. If not, a runoff election would be held within one month. A new government will take power May 29.
Electoral officials said they hoped to release results by late Monday.
Competition for government revenues flowing from the oil industry means Nigerian political seats are hotly -- and often violently -- contested. Despite the fact there were few reports of widespread violence Saturday, Abubakar's outright rejection of the election could undermine any ruling party win, pitting large segments of the population against the other.
Many voting centers opened well after the official start, if at all, and those that did were plagued by delays and what the opposition described as irregularities.
Local media described ruling party thugs running off with ballot boxes or driving away voters with guns and knives.
At one polling center in southern Nigeria, electoral workers could be heard shouting for the ballots to be hidden as an AP reporter approached. A week earlier, in the same area, workers could be seen affixing their own thumbprints on stacks of ballots and stuffing them into the boxes.
Presidential ballots distributed Saturday in many parts of the country lacked serial numbers that would guard against fraud by allowing officials to track the papers from ballot boxes through collation centers.
Iwu said there was no time to print serialized ballots as Abubakar's name was only added to the ballots during the week after a Supreme Court ruling clearing his participation in the vote.
In the main city of Lagos, some polling centers in opposition strongholds had only a fraction of the ballot papers needed, sparking accusations that the vote was being suppressed. Electoral workers freely reached hands inside ballot boxes.
Obasanjo, a former military ruler, won a 1999 election that ended 15 years of near-constant military rule. His 2003 re-election was marked by allegations of massive vote rigging. He was prevented from running again by constitutional term limits.
This campaign, as in 2003, has been troubled. The opposition rejected as fraudulent elections a week earlier for state offices largely won by Obasanjo's ruling party.

--The Associated Press