Amnesty International says at least 20 people have died in political attacks and clashes in Nigeria over the past two weeks.
A series of national polls is to begin on Saturday.
Voting will be held over three weekends - first in legislative, then presidential and lastly in state polls. Seventy four million people are registered to vote.
It is the third time that general elections are being held since military rule ended in 1999.
The BBC reports the previous ones in 2003 and 2007, were marred by widespread rigging, voter intimidation and ballot vote snatching.
Amnesty International says some of the worst trouble has been in the southern state of Akwa Ibom.