Nigeria's electoral commission says some of its staff were killed, injured and kidnapped during an election re-run on Saturday in Rivers state.
An Independent National Electoral Commission (Inec) spokesman blamed the violence on "armed thugs... allegedly acting on behalf of some politicians".
Voting was suspended in most areas of the oil-rich state, which has suffered from political unrest in the past.
A re-run was ordered after legal disputes over elections in 2015.
Elections in the state are seen as a battle for the control of Nigeria's largest oil wells.
Voters were choosing seats for the state and national assemblies, but not the governor as the Supreme Court ruled his election last March should stand.
Several other deaths were reported in the polls, which have now been indefinitely suspended.
In a statement lamenting the "deviant behaviour" of those involved in disrupting the polls, Inec spokesman Oluwole Osaze-Uzzi described "fatalities, kidnappings, [and] ballot snatching", among other offences, which forced the vote's suspension.
Results in areas which had already been declared would stand, he said.
Despite River state's huge resource wealth, it remains poor and underdeveloped for the majority of the communities who live there.
There is huge environmental pollution in some parts of the state due to oil spills.
Rivers state has long been a flashpoint for political violence with the two leading parties - the Peoples Democratic Party (PDP) and All Progressives Congress (APC) blaming each other for the friction.