LAGOS - Nigeria on Sunday anxiously awaited results from closely fought regional elections, with the main opposition headed for further gains after its historic win in the presidential vote last month. The All Progressives Congress (APC) said it had won the governor's seat in the northern state of Kaduna, which had been controlled by President Goodluck Jonathan's Peoples Democratic Party (PDP) since the end of military rule in 1999.

A total of 29 governorship and deputy governorship positions from Nigeria's 36 states are up for grabs as well as seats in all of the states' legislatures.

The APC, which takes power nationally when president-elect Muhammadu Buhari is sworn in on May 29, is eyeing a series of other PDP-held states as it seeks to consolidate its position as Nigeria's dominant party after 16 years of much-criticised PDP rule.

Collation of results after Saturday's vote was also underway in the battleground of Lagos, where the PDP has hopes of winning the governorship race for the first time.

Partial and unofficial results reported by several local media outlets indicated the contest between the PDP's Jimi Agbaje and Akinwunmi Ambode of the APC was still too close to call.

Voting was extended into Sunday in the restive Rivers state after irregularities affected some polling stations in the southern oil-producing hub.

The APC's governorship candidate Nasir el-Rufai had earned 1,022,316 votes, more than double the tally of PDP governor Mukhtar Yero, an AFP correspondent at the collation centre said.

With two local governments yet to report, the result was not yet final, but El-Rufai's margin appeared unassailable.

His spokesman Kailani Mohammed told AFP that Yero had called to concede, information which could not be immediately confirmed from the governor's office.

The APC's apparent win in Kaduna, though not surprising, further highlighted the stunning gains in the 2015 election cycle made by an opposition party that was only formed in 2013.

The PDP controlled 21 states heading into Saturday's vote, but looked set to suffer further losses, including possibly in the religiously-divided Middle Belt region.

- Polling day violence -

The Independent National Electoral Commission (INEC) said nationwide polling on Saturday was marred by 66 incidents of violence, without giving further details or any casualty figures.

Rivers was the worst affected, with 16 separate outbreaks of unrest, an INEC statement said.

Tension in Rivers mounted in the run-up to the vote because of a personal rivalry between outgoing governor Rotimi Amaechi and Jonathan after the former's defection to the APC in 2013.

INEC's top official in Rivers, Gesila Khan, said voting has been extended in nine wards where election materials were never delivered to polling stations.

The results in other parts of the state have been thrown out after ballot papers were openly stolen, she added.

The APC alleged widespread rigging after Jonathan won Rivers with more than 95 percent in the national polls two weeks ago, insisting it would not allow a repeat in the state vote.

Amaechi's spokeswoman Ibim Semenitari on Sunday accused the PDP of orchestrating a massive ballot-stuffing campaign and called on the INEC to "ensure that the people's voices are not silenced". Despite the reported violence, INEC described Saturday's vote as "relatively peaceful".

Electronic voter identification devices - which were used for the first time in last month's general election and caused headaches in several states - broadly worked in the regional vote despite problems in some areas, INEC said.

Taraba in the east is in the spotlight because it could return a female governor for the first time in Nigeria's history in the shape of Aisha Jummai Al-Hassan.

Jonathan made history by conceding the presidential election before the final results were announced, winning plaudits for his statesmanship and for defusing the threat of violence.

Information Minister Patricia Akwashiki on Saturday urged candidates in the local polls to follow his example, with violence having hit previous elections in Nigeria.

"If our president was able to walk away from an election and accept defeat, everybody, the winner and the loser, should be able to accept that the people's will is what will carry the day," local media quoted her as saying.