Last week's state polls were marred by fraud and violence

The electoral commission head said the 60m ballot papers had been kept in South Africa to prevent tampering.

Correspondents say it is hard to see how papers can be delivered across the vast country in time for the start of polling - already delayed by two hours.

On Friday evening, militants attacked the government house in Bayelsa state.

It is understood the militants believed that was where the election material was being kept.

The oil-rich state in the insecure Niger Delta is home to Goodluck Jonathan, who as well as being state governor is the running-mate of the ruling PDP party presidential candidate.

The BBC's Abdullahi Kaura says people he has spoken to in Yenagoa, the capital of Bayelsa, say the streets are deserted and heavy machine gunfire and explosions can be heard.

NIGERIAN ELECTION 60m registered voters 120,000 ballot boxes 360 House of Representative seats to be elected 109 Senate seats to be elected 24 presidential candidates Main contenders: Atiku Abubakar for the AC, 60-years-old Muhammadu Buhari, ANPP, 64 Umaru Musa Yar'Adua, PDP, 55 Polls open 1000-1700 (local time) (0900-1600 GMT) To avoid a run-off, a candidate needs highest number of votes overall and at least 25% of votes in 24 of the 36 states Candidates' profiles Fears of chaos At the polls: Voters' log

Mr Jonathan was caught up in the clashes but was unhurt, AFP news agency reported.

There is already a strong military presence in the city and a military source told the BBC that reinforcements were o­n the way.

Our correspondent says there are two possible reasons for the violence.

Either the militants are angry that last week's state elections were rigged, or they have been denied money they had requested to pay for security for Saturday's polls.

The Niger Deltas has been restive for years with youths taking up arms to press for what they say is a fairer share of the oil drilled from their areas.

Militants have frequently kidnapped foreign oil workers.

Credibility problem

EU observers says they are concerned about the credibility of the vote after last weekend's flawed state elections.

After decades of mismanagement, Nigeria has a very poor road network, making it a mammoth task to distribute the ballot papers - from the deserts of the north to the swamps and creeks of the oil-rich Niger Delta.

The Independent National Election Commission (Inec) chairman said the 60m ballot papers had to be printed abroad because it was not possible to do this in Nigeria in just three days.

Maurice Iwu also said the start of polling would be delayed by two hours and would now begin at 1000 (0900 GMT).

However, the names of the presidential candidates are not o­n the papers - just the symbol of their political party.

On Monday, Vice-President Atiku Abubakar obtained a Supreme Court ruling that his name should be added to the presidential contest.

Surprise holiday

After violence and fraud in last week's state elections, Mr Abubakar and other opposition candidates called for the presidential poll to be postponed.

The more thugs you have, the more votes you have Sha'aya'u Aminu, 23, student Voters' views Using texts to monitor polls At the polls: Reporters' log Nigeria fears raise oil prices

He said he was o­nly taking part in order to be able to challenge the results in court.

President Olusegun Obasanjo has admitted there were flaws in the state polls and urged election officials to prevent rigging in the presidential vote.

Friday was declared a surprise public holiday, to allow people time to travel before the vote.

Many people would have turned up to work to find their offices closed, as the announcement came late o­n Thursday night.

Nigeria is Africa's most populous country and is o­ne of the world's biggest oil producers. It is of key strategic interest to both the West and the growing economies of the East.

But despite the country's huge oil wealth, tens of millions live in poverty without basic amenities.