Nigeria's opposition charged on Friday that rigging had already begun on the eve of a keenly-watched presidential election, but the authorities denied a report of doctored ballots.

The opposition Action Congress (AC) said troops in the north had intercepted a truck of completed ballots 24 hours before the poll, seen as a watershed for both Nigeria and Africa.

The electoral commission said the accusation, which followed widespread rigging in regional polls last week, could not be true because new ballots were still arriving from abroad after a last-minute change, and had not yet been distributed.

Opposition parties have declared no confidence in the commission after last week's state election, which handed the ruling People's Democratic Party (PDP) a landslide.

"Soldiers in Kaduna have intercepted a truck-load of ballot papers, all already thumb-printed for the PDP ahead of Saturday's elections," said the AC's Lai Mohammed, adding the ballots were being held at a barracks in Kaduna city.

But electoral commissioner Maurice Iwu told a press conference: "The ballot papers are arriving as we speak so they couldn't have been captured by anybody."

He announced a two-hour delay to 0900 GMT in the start of Saturday's voting, to give more time for ballots to be distributed. Many polling stations opened hours late in the regional poll.

The election is intended to mark a big step forward for democracy in Africa's most populous nation, ushering in the first handover from one elected president to another in a country scarred by three decades of army rule.

But observers witnessed every form of rigging at the regional poll, and said many of the results did not reflect the will of the people.

A last-minute Supreme Court ruling on Monday has deepened confusion over the election.

The court ruled the election commission acted illegally in disqualifying Atiku Abubakar, President Olusegun Obasanjo's estranged deputy and AC candidate. He was then reinstated.

Iwu said 65-million new ballots had been printed to include the Action Congress, but there had not been time to include the names and photographs of the candidates.

At least 50 people died in violence surrounding Saturday's election, and street protests greeted the announcement of results in many states. Troops killed 25 Islamic militants in northern Kano state on Wednesday after they attacked a police station and killed 13 officers.

The main opposition challenger, former army strongman Muhammadu Buhari of the All Nigeria People's Party, said the state poll was the worst election in Nigerian history.

In a nationwide address on Friday, President Olusegun Obasanjo conceded the first election was flawed, but told aggrieved losers to seek redress through the legal system.

The former military ruler said there was no alternative to democracy, which returned to Nigeria in 1999. "Let us continue to improve on the structure and the house rather than pull it down because it is leaking in part," he said.

Oil prices rose on Friday because of dealers' fears a sham vote could further disrupt supplies from the world's eighth largest exporter, where output is already cut by violence.

The PDP has fielded a little-known state governor, Umaru Yar'Adua, as its candidate but the opposition says he is a puppet. Critics say Obasanjo wants to continue to dominate from behind the scenes.