Opposition supporters took to the streets in parts of Nigeria as partial results from Saturday's regional polls showed the ruling party ahead.
Saturday's polling was marred by violence in some areas
In Bauchi in the north, demonstrators blocked the road to an election office. Other incidents were also reported.
The polls, for the country's 36 state governors and assemblies, were marred by reported irregularities and violence which left more than 20 people dead.
They were seen as a key test ahead of next weekend's presidential vote.
But the outcome of the regional polls is as important to many Nigerians as the presidential election.
State governors can be extremely powerful, controlling budgets of around $1bn, especially in oil-rich states.
'Protect our votes'
Partial results from election officials showed that the People's Democratic People (PDP) had won in most of the states declared so far.
President Olusegun Obasanjo, who heads the PDP, said he thought polling had gone relatively well, but the opposition said there were widespread irregularities and fraud.
Problems included polling stations that opened late, intimidation and the theft of ballot boxes, witnesses said.
The BBC's Alex Last saw men storming a polling station and leaving with ballot papers in the southern city of Port Harcourt.
There were also reports of attacks on police stations and election officials.
As results began coming in, street disturbances were reported.
"We are trying to protect our votes," said a man blocking the road in Bauchi.
"We know that Mallam (the opposition candidate) has won the election. We are here because we don't want the results to be tampered with," Reuters news agency quoted him as saying.
Other incidents were reported, including in the oil-rich Delta state, where the ruling party was named the winner. Armed youths blocked roads and burned houses, Reuters said.
The tension is rising, says the BBC's David Bamford in Abuja, and with presidential and parliamentary elections due next Saturday, there is not likely to be much relief for the authorities.
Nigerian police said at least 21 people had died in election-related incidents, but other reports put the toll far higher.
Up to 70 people were also killed in violence during pre-election campaigning, human rights groups say.