Nigerians returned to vote in governorship and state elections on Saturday after polling a fortnight ago was declared invalid in some areas because of violence.
The decision by Nigeria's electoral board to void March 9 elections in six states came two weeks after President Muhammadu Buhari won a second term in a poll denounced by his main rival as fraudulent.
The INEC, the National Electoral Commission, said violence and other irregularities had prompted voting to be cancelled in some areas of Kano and Sokoto, in the northwest, Bauchi and Adamawa in the northwest, and the central states of Benue and Plateau.
Another poll, in Rivers state, had been suspended.
Around half a million registered voters were eligible to cast their ballot Saturday.
Results are expected from early next week.
The governorship races in the six states are closely fought between the ruling All Progressives Congress (APC) and opposition Peoples Democratic Party (PDP).
State parliamentary elections were also taking place across the country.
Tempers have been frayed since Buhari's re-election, with the defeated PDP candidate Atiku Abubakar challenging the result in an election tribunal and claiming that results in several states were manipulated.
Election observers meanwhile criticised the organisation and running of the governorship and state elections, as well as presidential and parliamentary polls on February 23, citing reports of vote buying and intimidation.
Situation Room, an umbrella group of more than 70 civil society organisations monitoring the vote, has called for an independent inquiry into the entire election process.
The PDP denounced INEC's decision to halt voting in affected states, claiming it was "clearly leading the (governorship) race," accusing INEC of collusion with the ruling party.
INEC was "seeking ways to use the situation to aid the APC to alter the results and announce APC candidates as winners", it said in a statement Monday.
Buhari's APC won 13 states in the governorship elections, with the PDP winning 9.
Regional elections are fiercely contested in Nigeria, where governors are powerful and influential figures, controlling state finances and responsible for key areas from education to health.
Buhari will hope to consolidate his re-election victory whilst the opposition PDP seeks to claw back power at the state level.