Violence has left at least 14 dead in Nigeria's south as Nigerians go to the polls to choose state governors in the first of two elections intended to solidify civilian rule.

Police on Saturday said seven police officers were killed in two separate attacks by armed men in Port Harcourt, the troubled hub of Nigeria's oil industry.

Simmons, Al Jazeera correspondent, said: "A large number of armed men attacked a police station in the middle of the night, killing six officers and then setting fire to the building."

Two miles away, Simmons said a large number of armed men opened fire at another police station and used small amounts of explosives. One officer died in that attack.

Simmons said many Nigerians told him they were afraid to vote.

He said: "They told me, if police officers get attacked, what about us?"

Dozens of people have been killed in political violence in the months leading up to the poll, dozens of mostly opposition candidates have been disqualified and poor preparations have raised doubts about the credibility of the vote.

In other violence on Saturday, three troops were injured and armed men killed three political operatives elsewhere overnight in the region, police and military spokesmen said.

In a nearby state, fights between rival political supporters left four dead, Larry Hayford, a local journalist who was on the scene told the Associated Press.

Robert Rotberg, of the US Council on Foreign Relations, in a special report, said: "If Nigeria works well, so might Africa. If the democratic experiment in Nigeria stalls, the rest of Africa suffers and loses hope."

Nigeria returned to democracy in 1999 after three decades of almost continuous army rule.

Nigerian governors control big budgets and have enormous powers in their states, making the gubernatorial polls as important to many Nigerians as the presidential vote on April 21.

The ruling People's Democratic Party (PDP) now controls 28 of the 36 states, with the rest split between a handful of opposition parties.

With unrivalled funds and powers of incumbency, analysts say the PDP should coast to victory. But endemic corruption, failure to deliver basic services and deteriorating security have boosted the chances of the opposition in many states.

In eight years of democracy, more than 15,000 people have been killed in ethnic, religious and communal fighting - often stoked by politicians carving out territory for themselves.