By Tim Cocks
ABUJA (Reuters) - Results from Nigeria's elections, potentially the closest contest since the end of military rule in 1999, were due to start trickling in on Monday after a weekend vote marred by confusion, arguments and sporadic violence.
The election pits President Goodluck Jonathan against former military ruler Muhammadu Buhari for the favor of an electorate divided along a complex mix of ethnic, regional and in some cases religious lines in Africa's most populous nation.
Even before preliminary tallies were recorded, the opposition All Progressives Congress (APC) rejected the outcome in Rivers state, headquarters of Africa's biggest oil industry, and denounced the vote there as "a sham and a charade".
The INEC election commission said the first results collated from 120,000 polling stations nationwide should be available on Sunday evening although this failed to materialize.
Turnout among the 56.7 million registered voters appears to have been high.
The tension in Rivers state raises the prospect of a disputed national outcome and a repeat of the violence that erupted after the last election in 2011, when 800 people were killed and 65,000 displaced in the mainly Muslim north.
The APC reported violence in Rivers and blamed it on "armed militias" backed by the ruling People's Democratic Party (PDP). "Whatever trash will (be) announced as the result of today's election is not acceptable to us," it said.
The PDP did not respond to requests for comment.
An hour later, hundreds of sympathizers chanted "APC" outside the electoral commission office in the oil city of Port Harcourt, prompting police to fire warnings shots. One group stoned a car they thought carried ballots.
?"There was no election in Rivers," APC polling agent Achinike William-Wobodo said, calling for a re-vote.
In a sign the opposition will challenge results elsewhere, the APC governor of the southern Imo state, Rochas Okorocha, denounced on television the conduct of the election in his region and accused the military of meddling in the result.
INEC chairman Attahiru Jega said he was concerned about the Port Harcourt complaints, which alleged that opposition agents were kicked out of vote-tallying meetings, and had launched an investigation.
In Kaduna, the northern city worst-hit by the 2011 post-election violence, the streets were virtually devoid of traffic and many shops were shuttered.
(Additional reporting by Bate Felix; Writing by Ed Cropley; Editing by Janet McBride)