By Estelle Shirbon

ENAGOA, Nigeria, April 21 (Reuters) - At a polling station deep inside a slum in the Nigerian oil city of Yenagoa, an electoral official busily stuffed dozens of ballot papers into a ballot box.

All the voting slips for Nigeria's presidential election on Saturday, in which there were widespread reports of fraud, bore thumbprints next to the logo of the ruling People's Democratic Party (PDP).

Asked what was going on, the official said voters had marked the ballots earlier but she had not had time to put them in the ballot box. There were no voters in the room at the time.

On leaving the polling station, this reporter was led into a quiet alleyway by a group of young men who said they had thumb-printed dozens of votes for the PDP.

''I have thumb-printed more than 50 ballots,'' said one who gave his name as James, showing a thumb stained with purple ink.

The youths said men from the PDP came on Friday and gave 2 million naira (,000) to share among the community. Now these youths were angry because they had not been paid for their work.

''Some people have taken all the money and they don't want to share,'' said one man called Thankgod, lowering his voice and looking over his shoulder.

''This is not an election. The only people who go ... are going to thumb-print for the PDP. If you try and vote for AC they will drive you away,'' he said, referring to the opposition Action Congress.

President Olusegun Obasanjo denied any vote tampering took place. ''I want to assure Nigerians that this government is a law abiding government. This government has no reason to tamper with election results,'' Obasanjo said after casting his vote in Abeokuta in southwest Ogun state.

Yenagoa is the capital of Bayelsa, one of three major oil-producing states in the Niger Delta, an impoverished wetlands region in southern Nigeria despite being the source of most of Nigeria's wealth.

MILITANT RAIDS All three are plagued by frequent militant raids on oil facilities, abductions for ransom and shoot-outs between local militias and troops.

The PDP is fielding Goodluck Jonathan, the governor of Bayelsa, as its vice-presidential candidate.

''I want Goodluck to be the VP because he will bring development. We have no power, no water, no motor roads.

Goodluck will bring all these things to us,'' said Inimotimi Amos, a young trader in Yenagoa.

Such sentiments would likely have secured the state for the PDP by fair means, but residents and activists said the vote was being manipulated anyway.

''The polling stations are to give the impression that an election has taken place, but the election proper was done yesterday at government house,'' said Ifeanyi Jonjon of the Ijaw Youth Council, which campaigns for the rights of the Niger Delta's majority ethnic group.

''The youths see that the politicians use this kind of kangaroo election to impose themselves on the people and take control of the resources,'' he said.

''The effect is that they also see it as their right to carry guns and hijack anything they want.'' The night before the election, gunmen tried to attack state government headquarters and waged a two-hour battle with security forces in the city centre. Part of a hotel just 300 metres (yards) from the headquarters was reduced to rubble.

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