The APC's Babajide Sanwo-Olu scored a landslide re-election as Lagos governor, the Independent National Electoral Commission (INEC) said on Monday.
The incumbent won more than twice the number of votes of LP candidate Gbadebo Rhodes-Vivour.
Governors are powerful figures in Nigeria and some control state budgets that are larger than those of several African nations.
Saturday's elections for 28 governors and more than 900 state assembly lawmakers came three weeks after the governing party won a presidential ballot that opposition candidates claim was massively rigged.
So far, the APC has won the governorship races in the states of Gombe, Jigawa, Katsina, Niger, Kwara, Nasawara, Ogun, Sokoto and Yobe.
With the count ongoing, the ruling All Progressives Congress (APC) was declared winner in the majority of governorships where tallies had been completed.
- 'A sense of discouragement' -
The People's Democratic Party (PDP) has won in Oyo, Delta, Bauchi, Plateau and Akwa Ibom.
In Kano, the economic heart of the mainly Muslim north, thousands of supporters of the smaller New Nigeria People's Party took to the streets to celebrate their trouncing the APC.
Results were still pending on Monday in the key southern state of Rivers. Adamawa, in the northeast, could see the election of Nigeria's first woman governor.
The Center for Democracy and Development (CDD), which observed Saturday's poll, noted "a sense of discouragement that due to the unfavorable outcome of the presidential election 'there is no point' coming out to vote".
Polling units mostly opened on time and both the biometric registration machines and online portal to view results functioned relatively well, the CDD and another observer group, Yiaga Africa, said.
- Beatings and disruption -
But violence was recorded across several states, with thugs showing up at polling units to intimidate voters and in some cases destroying electoral material.
Voting was postponed until Sunday at some polling stations in Lagos and Rivers State as a result of tensions.
"Multiple incidents of thuggery and intimidation interrupted polling in various locations," the EU observation mission said on Monday. "Vote buying, also observed by EU EOM observers, further detracted from appropriate conduct of the elections."
In southeast Imo State, where armed separatist groups are active, a group of ad hoc electoral staff were taken hostage on Saturday morning. They were quickly rescued but election material went missing.
Almost 100 electoral staff, volunteers and their drivers came under gunfire while driving to a ballot collation center in Kogi on Sunday, police said. Three people were injured before police reinforcements managed to rescue them.
The INEC also suspended vote counting in some parts of Abia and Enugu states in the southeast after thugs tried to invade ballot collation centres.
"Processes were disrupted by actors over whom we have little or no control," INEC official Festus Okoye said on Sunday evening, condemning the violence.
Vote buying was more rampant than during the presidential election, observers reported earlier in the day.
Party agents were seen giving out 1,000 naira (about $2) in exchange for votes, as well as spaghetti, fabric and alcohol, Yiaga Africa said.
