(Updates with details on Bayelsa attack) By Tom Ashby ABUJA, April 20 (Reuters) - Militants attacked government buildings in Nigeria's oil region on Friday, a few hours before a presidential election in which opposition parties alleged vote-rigging had already begun. The attack appeared clearly timed to coincide with Saturday's election in Africa's most populous nation, the eighth-largest oil exporter in the world. Bayelsa state governor Goodluck Jonathan is vice-presidential candidate for Nigeria's ruling People's Democratic Party (PDP), which the opposition accuses of preparing to rig the election. Reuters photographer George Esiri reported heavy gunfire and explosions near Bayelsa state government house in Yenagoa. "The ground is shaking," he said. There was no immediate word on casualties. Bayelsa is home of the Ijaw ethnic group, which has been at the forefront of an insurgency against the federal government to demand more autonomy and a greater share of big oil revenues. Militant attacks on Western oil installations have forced Nigeria to reduce crude exports by a fifth for more than a year. The new Bayelsa violence was sure to increase jitters ahead of Saturday's vote that pushed up oil prices on Friday. An Ijaw activist said he had spoken to militants involved in the attack who said they were angry about the outcome of the election for state governor in Bayelsa a week ago, in which the ruling party's Timipre Sylva emerged victorious. "They said there was no voting in Ijawland," the activist said, asking not to be named. The attackers favoured a rival candidate for Bayelsa state governor, he added. Opposition parties charged earlier on Friday that rigging had begun on the eve of the presidential race, but authorities denied ballots had been doctored. BALLOTS INTERCEPTED The opposition said troops in the northern city of Kaduna had intercepted a truck of ballots marked up for the PDP. In nearby Katsina, opposition parties accused the PDP of doctoring ballots after taking them to private houses, and of masterminding the arrest of hundreds of their sympathisers. Regional polls last weekend, which gave the PDP a landslide, were marred by widespread abuses. "Soldiers in Kaduna have intercepted a truck-load of ballot papers, all already thumb-printed for the PDP ahead of Saturday's elections," said Lai Mohammed, spokesman for the Action Congress (AC) party. The electoral commission said in the morning ballot-tampering could not have occurred because new voting slips were still arriving from abroad after a last-minute change, and had not yet been distributed. Electoral commissioner Maurice Iwu announced a two-hour delay to 0900 GMT in the start of Saturday's voting, to give more time for ballots to reach far corners of the vast, chaotic nation, which is Africa's top oil producer. The election is intended to mark a big step forward for democracy, ushering in the first handover from one elected president to another in a nation scarred by three decades of army rule. But observers witnessed every form of rigging at last Saturday's regional election, and said many of the results did not reflect the will of the people. At least 50 people died in violence surrounding the regional election, with street protests against many results. 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.
