CDD also said online and offline voter suppression affected the governorship elections.
The Centre for Democracy and Development (CDD) has identified vote buying and violence as some of the factors that characterised the governorship elections in many states.
In its interim report on the elections on Saturday, the pro-democracy think-tank said "those who are participating appear to be more open to inducement for casting their ballot with vote trading being undertaken by all political parties across Nigeria's six geo-political zones."
On violence, it said: "Thuggery has been used to disrupt the process in different parts of the country, with repeated examples of this in the South-south."
The organisation, however, said it observed that the Independent Electoral Commission (INEC) improved its logistics for the election.
Read the full report here.