* Ballot box snatching, violence mars governorship polls
* Most violence recorded in oil hub Rivers state
* Voting extended to Sunday in affected places
* Protesters in Akwa-Ibom state call for cancellation (Adds Akwa-Ibom protest, details on vote irregularities, context)
By Julia Payne and Anamesere Igboeroteonwu
LAGOS/ONITSHA, Nigeria, April 12 (Reuters) - Voting for Nigeria's powerful state governors was extended on Sunday in several states after ballot boxes were snatched and violence broke out, particularly in Rivers state, the country's oil hub, electoral commission officials said.
The 36 governors are among the most influential politicians in Nigeria, Africa's biggest oil producer and economy, with budgets larger than those of small nations.
Observers and voters said fears of violence and apathy had made the turnout to elect 29 governors, and all state assemblies on Saturday low compared with the presidential vote last month, which was considered the freest and fairest yet and has paved the way for Nigeria's first democratic transfer of power.
The Independent National Electoral Commission (INEC) in Rivers said the vote was cancelled in all places where materials were taken. "The election in some units of 6 councils was inconclusive and a fresh election will be held today," Rivers resident electoral commissioner Gesila Khana told journalists.
In the presidential poll, Muhammadu Buhari of the All Progressives Congress (APC) beat President Goodluck Jonathan of the People's Democratic Party (PDP) by a landslide.
The PDP has dominated politics since the end of military rule in 1999 and tensions are particularly high in Rivers as current governor Rotimi Amaechi has been feuding with Jonathan since defecting to the APC two years ago.
A large protest in state capital Port Harcourt, shoot-outs in several towns and attacks on INEC property delayed the start of voting in the state on Saturday and again on Sunday. Amaechi called the polls a "sham".
INEC headquarters in the capital Abuja said the election went "very well" across the country but 5.2 percent of polling units did not open until 1 p.m. (1200 GMT) on Saturday.
There were also 66 instances of violence at polling units, with the highest number seen in southern and south-eastern states. Rivers had the highest incident rate at 16, followed by Ondo, Cross River, Ebonyi and Akwa Ibom.
On Saturday, at least 10 people were killed in election-related violence including the PDP chairman of a government area of Ebonyi State who police said was shot in his house on Saturday.
More than a dozen people were killed during the presidential polls, mainly due to attacks by Islamist extremist group Boko Haram, which has been waging a six-year insurgency in the northeast of the country.
HIGH STAKES, PROTEST IN SOUTH-EAST
In Lagos, the stakes are even higher than in Rivers, though the polling was calm. The state holds the bustling commercial capital of 21 million people and has an economy twice the size of Kenya's. Lagosians have traditionally voted for the opposition. Results from 14 local government areas show a tight race, but so far in the APC's favour.
Outgoing Lagos APC governor Babatunde Fashola is credited with transforming the metropolis with infrastructure projects, although he has also been criticised for slum clearance.
In the south-eastern state of Akwa-Ibom, tensions flared on Sunday with a gathering of APC members and sympathizers protesting in front of INEC headquarters in state capital Uyo, demanding a fresh election.
"The protesting crowd numbered over 150 ... there are burnt tires along the Udo Uduma avenue and street traffic lights were destroyed. There is a heavy police presence," witness Edet Udo told Reuters.
Polling in a few other states was also extended or was being debated, while collation and counting was still on-going in many others. Some results have started trickling in.
Borno state INEC spokesman Tommy Magbuin said polls re-opened in three local government areas in the northeastern state after ballots were not delivered in time.
In the south-eastern state of Imo, ballot boxes were stolen in three districts, which could mean a vote extension. Elections in some areas of Abia also had to be cancelled due to ballot box theft and southern Ondo state also experienced ballot snatching.
In Anambra, where only state assembly elections were taking place, polls in one area will be pushed back due to irregularities and violence, INEC officials said.
In Bayelsa, also only holding state assembly elections, the vote was rescheduled in eight out of 24 constituencies and other areas could be re-done due to skirmishes. (Additional reporting by Tife Owolabi in Port Harcourt, Buhari Bello in Jos and Lanre Ola in Maiduguri; editing by Philippa Fletcher)