* President praises conduct of poll
* Vote seen as test before national elections in April
* Opposition candidate complains of irregularities
(Recasts with result and comment from president)
By Samuel Tife
WARRI, Nigeria, Jan 7 (Reuters) - The governor of one of Nigeria's main oil states was declared the winner on Friday of an election seen as a litmus test for national polls in April, but his opponent questioned the conduct of the vote.
Emmanuel Uduaghan of the ruling People's Democratic Party (PDP) was declared re-elected in Thursday's poll, which was held after a court last year overturned his 2007 election victory.
Delta is one of three main states in the Niger Delta, the heartland of Africa's biggest oil and gas industry and a restive region seen as a potential flashpoint during the presidential and parliamentary elections in three months' time.
President Goodluck Jonathan, who faces a tough battle in the presidential race, congratulated Uduaghan and said Thursday's vote had been a vast improvement on previous elections.
"INEC (the electoral commission) has shown by its performance in Delta state that Nigeria is well on its way to achieving very credible general elections in April," Jonathan's office said in a statement after the results were announced.
Thousands of armed police and soldiers were drafted into Delta state for the vote, but despite isolated acts of sabotage and intimidation, there was no major unrest. [ID:nLDE7050WD]
Uduaghan won by a considerable margin, polling just over 275,000 votes compared to 138,000 for his nearest rival, Great Ogboru of the Democratic People's Party (DPP).
Ogboru earlier hinted he might not accept the result if Uduaghan won. He said some of his party agents had been unfairly arrested and he had doubts about how the election was conducted.
"This injustice must not be allowed to continue because of major irregularities in some (voting) areas," Ogboru told journalists in the state capital Asaba.
Military trucks carrying hundreds of soldiers arrived in the main oil city of Warri on Friday and were patrolling the streets, although there were no immediate signs of unrest.
The electoral commission has acknowledged there were some problems, including omissions on the voters' register and the late arrival of election materials, but said they were not enough to call the credibility of the poll into question.
Nigeria has been shaken by violence in recent weeks, including a New Year's eve bomb blast near an army barracks in Abuja a week after a series of blasts and subsequent clashes killed 80 in the central city of Jos. [ID:nLDE70101O]
Political rallies have turned violent in some areas.
Africa's most populous nation wants to avoid a repeat of chaotic polls in 2007, including the one that originally brought Uduaghan to power, which were so marred by intimidation and fraud as to be deemed not credible by observers. (Writing by Nick Tattersall and Joe Brock; editing by Andrew Roche)
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