Police said four people were killed and 22 arrests were made at the weekend for disrupting the electoral process, which INEC said included the barricading of its offices.
"Of more serious concern was the level of threats, violence and intimidation of election officials and voters by well-armed thugs and miscreants allegedly acting on behalf of some politicians, which marred the elections in some areas," it added.
"There were reports of numerous attacks resulting in fatalities, kidnapping, ballot snatching, diversion of officials and materials, amongst others."
As a result of the violence and intimidation, results were only declared in one federal and nine state constituencies, he added.
Also Read
Re-runs of last year's elections for the Rivers state assembly and the national assembly in Abuja were ordered by the courts because of repeated claims of widespread irregularities.
Rivers has long been a key battleground because of its location in the heart of the oil-producing south and its major urban centre, Port Harcourt, the state capital.
Both sides have blamed the other for a spate of politically motivated killings and violence.
Eneruvie Enakoko, a Nigerian writer and campaigner on social issues including political freedom, said the situation was a bad advertisement for Nigeria and had made the country "a laughing stock".