* Early results underscore ethnic, religious divide
* Contest between President Jonathan, former army ruler
* Close contest could lead to second round vote
ABUJA, April 17 (Reuters) - Results emerging slowly from Nigeria's presidential election on Sunday showed a close contest, stoking tension in northern opposition strongholds where anxious youths feared a plot to rig the count.
Votes were almost entirely split between pre-poll favourite President Goodluck Jonathan, a Christian from the oil-producing Niger Delta, and former military ruler Muhammadu Buhari, from the dustblown Muslim north.
Tens of millions voted on Saturday in what was tentatively described by observers as the most credible presidential election for decades in Africa's most populous nation.
The drip of results from 120,000 polling stations underscored Nigeria's divisions.
Jonathan appeared to have done very well in much of the south, even in places such as the sprawling commercial hub of Lagos where his ruling People's Democratic Party had struggled in parliamentary elections a week earlier.
But Buhari had a massive lead in figures from polling stations across heavily populated northern states, where turnout also appeared to have been very high.
"We are walking a tightrope," said Yusuf Tuggar, from Buhari's Congress for Progressive Change and a candidate for governor in northern Bauchi State in an April 26 ballot. "People are getting excited." <^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ Reuters Insider preview: link.reuters.com/rah98r Graphic on elections: link.reuters.com/xet78r For more stories, background and analysis: [nLDE68H051] ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^>
To avoid a run-off, a candidate has to get a simple majority and at least a quarter of the vote in two-thirds of Nigeria's 36 states. With more than 73 million registered voters, a final tally could take days to emerge.
Fearing the ruling party would try to fiddle the results -- the norm in ballots since army rule ended in 1999 -- Buhari supporters massed on the streets in northern cities.
Police were investigating a possible bomb blast in a brothel near a polling station in the city of Kaduna. The house of a local ruling party official was burned down in the town of Azere.
Shots were fired in Bauchi and a car suspected to be carrying fraudulent ballots was set ablaze in what turned out to be a case of mistaken identity. Youths stalked the streets armed with bows and arrows.
Jonathan is the first head of state from the main oil producing region of Africa's biggest crude exporter. Should he become the first sitting president to lose an election, there could be trouble in his already volatile home region.
Turnout for the presidential election was high in the Niger Delta, where people often did not bother to vote in the past because they knew results would be rigged and feared intimidation by heavily armed thugs.
"It is a path that would have been hard to imagine four years ago or even two years ago," said Chris Newsom, an adviser to civil society group Stakeholder Democracy Network in the Niger Delta.
Buhari's support was bolstered by a feeling among many in the north that Jonathan is usurping their right to another four years in power. Jonathan inherited office after his predecessor, northerner Umaru Yar'Adua, died last year in his first term, interrupting a rotation between north and south.
Although a second round of voting could deepen uncertainty and worsen Nigeria's polarisation, it could also be a mark of deepening democracy in a country that sets an example for the rest of Africa.
"If it went to a second round that would be a massive achievement in a system that heavily favours the incumbent," said Antony Goldman of PM Consulting. "In the previous elections it was inconceivable."
(Additional reporting by Nick Tattersall)
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