Lagos - The final stage of Nigeria's election began on Saturday with voting for powerful state governors, two weeks after a presidential poll saw an incumbent leader unseated at the ballot box for the first time.
The 36 governors are among the most influential politicians in Africa's biggest oil producer and top economy, controlling budgets larger than those of small African nations and wielding influence that can decide who goes on presidential tickets.
Voting started in some states shortly after 07:00 GMT, although in many others it had yet to begin.
With so much at stake, candidates in past governorship elections have often played dirty, snatching ballot boxes, manipulating voter turn-out and engaging in thuggery and intimidation.
Police fired shots into the air to clear an angry mob of 2 000 people in the oil hub of Port Harcourt, in hotly contested Rivers state, after voting materials did not arrive on time, a Reuters witness said.
Muhammadu Buhari beat President Goodluck Jonathan last month with 15.4 million votes against 13.3 million, in a vote that was considered free and less violent than past polls.
Yet for many Nigerians, who their governor is matters more than who sits in the faraway capital Abuja.
"It will be slightly dirtier, a bit rougher in some places. Locals are more invested in the process around them. The federal government is like a no-man's land," said Folarin Gbadebo-Smith, head of Nigeria's Centre for Public Policy Alternatives.
Legislative elections also shifted power away from the Jonathan's People's Democratic Party (PDP), which has run Nigeria since the end of military rule in 1999, to Buhari's All Progressives Congress (APC). It left the APC with a majority in both houses.
The APC is seeking to build on its gains, while the PDP will hope it can claw back power, especially in two battlegrounds - the megacity of Lagos, Nigeria's economic engine generating up to a third of its GDP, and the oil hub of Rivers.
Both are currently APC, Rivers only because its sitting governor defected. Both could be magnets for trouble, as could the northern swing state of Kaduna, which saw hundreds killed in sectarian violence after the 2011 presidential election.
In all, 29 states were holding a vote, with the rest sitting them out following by-elections in the past few years. The new governors will take office on May 29, as will Buhari.
"The Nigerian army has directed all units to remain vigilant... to prevent any attempt to endanger the lives of Nigerians," the army said in a statement on Friday.
Biometric voting cards that were used for the first time in last month's vote, and helped prevent fraudulent practices like multiple voting or ballot box stuffing, were again being used.
"The card reader is the only way that rigging on a large scale can be stopped in this country," Abubakar Momoh, of the Independent National Electoral Commission (INEC), told Reuters.
Outgoing APC Lagos governor Babatunde Fashola is credited with transforming a megacity of some 21 million people and an economy twice the size of Kenya's through infrastructure projects, although he has also been criticised for slum clearance