By Barry Moody

ABUJA, April 22 (Reuters) - Nigeria's presidential election was tainted by violence and fraud, a far cry from the credible democratic vote many had hoped would mark the country's first handover from one civilian president to another.

Optimism was swiftly dashed that Africa's most populous nation would cement democracy on Saturday after decades of corrupt military rule, which looted Nigeria's oil riches and left most of the population in grinding poverty.

Little known state governor Umaru Yar'Adua of the ruling People's Democratic Party (PDP), President Olusegun Obasanjo's chosen successor, was expected to win an overwhelming victory.

But opposition leader Muhammadu Buhari said no-one could claim to have won an election with so many flaws. He said he was likely to call his supporters onto the streets if the PDP announced victory.

"It is not a question of winning because I don't think there have been elections," Buhari said.

Troops opened fire in northern Daura, Buhari's hometown, when hundreds of youths smashed cars and set fire to roadside shacks after thousands of ballots were reported missing.

Three boys aged between 11 and 17 died and 10 other people were injured, hospital sources said.

Thugs armed with cutlasses and guns stole ballot boxes in northern Kano and an election official was kidnapped in southwestern Ondo state.

State media said nine policemen were killed by highway robbers on Friday on their way to election duties in central Nasarawa state.

European Union observer Max van den Berg said he was unsure there had been any improvement over regional polls last week, when there was widespread fraud and 50 people were killed. "For the moment I am worried," he told Reuters. Clear results are not expected before Monday.

TANKER BOMB

Hours before polling stations opened on Saturday, somebody tried to blow up the national electoral headquarters in the capital with a petrol tanker. It hit a telephone pole outside the building and failed to explode.

Reuters correspondent Estelle Shirbon saw national electoral commission officials in southern Bayelsa state stuffing dozens of completed votes into ballot boxes.

A young man, who gave his name as James, complained he had thumb-printed 50 ballots for the ruling People's Democratic Party (PDP) but had not been paid.

Obasanjo, who the opposition accuses of trying to install Yar'Adua as a puppet, rejected accusations of wholesale rigging.

"I want to assure Nigerians that this government is a law abiding government. This government has no reason to tamper with election results," he said.

Obasanjo has failed to change the constitution to give himself a third term.

He came to power in 1999 when democracy returned following three decades of almost continuous army rule, but Nigerians are angry that corrupt politicians continue to loot the nation's riches and keep most of the population poor.

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