Suspected Boko Haram extremists attacked polling stations and destroyed election material in two northeastern towns, ABC news reported.
Fleeing residents said scores of the extremists headed for Nigeria's northeastern city of Bauchi and soldiers engaged them with heavy gunfire.
Police spokesman Haruna Muhammad said security forces had stopped a convoy of 10 vehicles of "unidentified gunmen" at Dindima village, 10 kilometers (6 miles) from Bauchi.
Muhammad said the gunmen attacked polling stations in Kirfi and Alkaleri towns earlier Sunday.
In the southern oil-producing center of Port Harcourt, thousands of Nigerians took to the streets to protest alleged killings of opposition campaign workers and voting irregularities. Police in riot gear and armored cars moved in to control the demonstration.
Police reported that three people including a soldier were shot and killed in Port Harcourt Saturday.