Port Harcourt - Voting has been extended into Sunday in Rivers state after irregularities at some polling stations, officials said.
The extension affects nine wards in Rivers, said the state's top electoral officer Gesila Khan.
She said that in some instances key election materials were never brought to polling stations.
The results from Saturday's election have been cancelled in other parts of the state after ballot papers were openly stolen, she added.
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Khan did not specify how many areas were affected by cancellations and so it was not immediately possible to estimate the impact on the statewide result.
Rivers has emerged as the key battleground in the 2015 regional elections, after the state saw unrest during presidential polls last month.
Tensions has been fuelled by personal rivalry between outgoing governor Rotimi Amaechi and President Goodluck Jonathan after the former's defection to the opposition All Progressives Congress in 2013.
The APC alleged widespread rigging on the part of Jonathan's People's Democratic Party after polls opened in Rivers on Saturday.