Nigerians began voting to elect new governors in two states on Saturday with security forces out in numbers to prevent any violence following pre-vote attacks.
The two states, southern oil-rich Bayelsa and central Kogi, are among seven states where gubernatorial elections are held at different times from the general election due to court rulings.
Bayelsa has been ruled by the main opposition People's Democratic Party since Nigeria returned to civilian rule in 1999.
Voting opened late in most polling stations in the state, with long queues of voters forming in Yenagoa, the state capital, and elsewhere.
The polls were officially to close at 2:00 pm (1300 GMT), but electoral officials said everybody queuing would be allowed to vote even after the deadline.
The Independent Electoral Commission of Nigeria (INEC) said over 31,000 police had been deployed, as well as 87 gunboats, to prevent, or deal with, any electoral violence.
This week, a staffer at a radio station was shot dead and many injured during an attack on a political rally in Bayelsa, and in Kogi state a campaign office was burnt down.
"Already there have been several instances of violence at election campaign rallies in both Bayelsa and Kogi states," Amnesty International said on Friday.
"Amnesty International has received reports of supporters of some politicians violently targeting political opponents, real or perceived," it said.
Voters said they hoped for a peaceful election day.
"We crave a free, fair and hitch-free elections in Bayelsa. Everybody should be allowed to exercise his or her franchise without harassment and intimidation," Joseph Cookey, a textile trader in the southern city of Port Harcourt, told AFP.
Housewife Alice Ebere urged "politicians to shun violence and allow the wish of people to prevail".