The top opposition candidate in Nigeria's presidential election, along with an election monitoring group, called on Sunday for the vote to be annulled amid allegations of voting irregularities and fraud.

Vice-president Atiku Abubakar, one of the three leading contenders and leader of the Action Congress party, branded Saturday's presidential and legislative elections "a mess."

"I have already rejected the elections. They have no alternative other than to cancel them altogether," Abubakar told reporters in the capital, Abuja. "What we have seen clearly proves our fears that it is the worst election ever seen."

Many voting centres opened well after the official start, if at all, and those that did were plagued by delays.

At several several polling stations in Lagos, there were more voters than ballots, said CBC's David McGuffin, reporting from that city.

"In the southeast of the country, in the Niger River delta, home to an ongoing growing insurrection, there were polls that didn't open, there were explosions outside some polling stations and there were reports of ballot boxes being stolen," McGuffin said.

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 Associated Press reported ballot-paper shortages in opposition strongholds and open rigging favouring the ruling party of outgoing President Olusegun Obasanjo. Local media reported intimidation by thugs with knives and guns.

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.

With files from the Associated Press