The ruling All Progressives Congress is declared the winner in the majority of the states where the vote count has been completed.
Nigeria's two biggest parties won the majority of states in weekend governorship polls, official figures have shown, maintaining their political dominance following elections in which European Union observers said 21 people died from violence.
Barry Andrews, European Union Observer Mission Nigeria chief observer, told a news conference on Monday that commercial hub Lagos was among several states disrupted by multiple incidences of thuggery, ballot snatching and intimidation of voters, polling officials, observers and journalists.
Voters were choosing governors in 28 of the country's 36 states to bring to an end this year's election cycle that began with disputed presidential and legislative elections last month.
Ruling All Progressives Congress (APC) party's Bola Tinubu was declared president-elect but his victory is being challenged by Atiku Abubakar of the main opposition People's Democratic Party (PDP) and Labour Party's Peter Obi.
Obi was supported by young voters and was seen as having the best chance to end the dominance of APC and PDP, which have between them, ruled Africa's most populous nation since the return to democratic rule in 1999.
But Obi came third in the presidential race behind Tinubu and Atiku and his Labour Party was yet to win a governorship race.
Official figures showed that APC won 15 states, including the closely watched Lagos race, compared to seven for PDP, which led in another state as counting continued. A northern regional party won one state and led in another.
READ MORE: Nigerians vote in high-stake governorship polls with Lagos in focus
Labour Party, which rejected the Lagos result, led in one state.
Two races were declared inconclusive because they were too close to call between APC and PDP. One was northeastern Adamawa state where the APC candidate was tipped to become the first female elected governor.
Andrews said violence and intimidation may have led to a low turnout.
While there is no nationwide turnout figure, because the voting is administered state by state, authorities put it at 16 percent in Lagos - Nigeria's most populous with more than 20 million people.
"It's actually tragic that 21 people, according to our latest information, lost their lives in election-related violence," he said.
Andrews said, however, there were some improvements in Saturday's vote.
Election materials arrived on time at most polling units, which opened early while electronic voting machines worked better than last month.
The number of reported deaths was below the final tolls in previous Nigerian elections. Most unrest typically occurs after results are announced.
READ MORE: Nigeria begins vote count for state governors in shadow of poll violence