Voting in Nigeria's regional elections extended into Sunday in some parts of the country after irregularities were reported in several districts, particularly in the oil hub Rivers state.
Voters were electing 36 state governors, with 29 of the races contested. Turnout appeared to be weaker than in the presidential vote, when President Goodluck Jonathan became the first head of state voted out of office in a peaceful change of power.
Officials at the Independent National Electoral Commission (INEC) headquarters in the capital, Abuja, said voting on Saturday was marred by 66 incidents of violence at polling places, with the highest number seen in southern and southeastern states.
Election monitors reported the seven people were killed in Rivers state, where it was also said that an INEC office in Buguma was dynamited.
"These killings form part of a wider pattern of politically motivated violence, arson, and hijacking of electoral materials that our observers have noted,” the Nigeria Civil Society Situation Room said in a statement. It added that electoral regulations at many polling stations were "flagrantly flouted."
Eyewitnesses told Reuters that shootings on the streets of several Rivers towns forced voters to initially remain indoors and that voting began hours late after a 2,000-strong early morning protest in the state capital, Port Harcourt, delayed the distribution of materials.
Violence was also reported in Lagos, Delta, Ebonyi and Akwa states.
Nigeria's 36 state governors are among the most powerful political figures in the nation that is Africa's biggest oil producer and top economy.
Analysts said with so much at stake, candidates in past gubernatorial elections have often played dirty, snatching ballot boxes, manipulating voter turnout and engaging in thuggery and intimidation.
The U.S. State Department called on Nigeria's political leaders Friday to emphasize the message to their supporters that they should remain peaceful and pursue any grievances through the judicial process.
Some material for this report came from AFP and Reuters.