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.
Voting began hours late on Saturday in many polling stations in the three major cities of Lagos, Port Harcourt and Kano. Turnout was uneven and many voters complained of logistical problems.
In the northern city of Kano, where a Muslim cleric was assassinated in a mosque on Friday, many polling stations opened about two hours late and voters complained of a lack of secrecy.
In Port Harcourt, Simmons said some polling stations opened three hours late, others didn't open at all and "voting papers haven't arrived in areas".
Simmons said many Nigerians told him they were afraid to vote.
Under Nigeria's federal constitution, the president and 36 state governors are elected directly to a four-year term, for a maximum of two terms.
There are 24 parties offering candidates for president and almost 50 parties in total. The three largest parties are: People's Democratic Party, All Nigeria People's Party and Action Congress.
April 14 - Election of governors for 36 states and members of state houses of assembly.
April 21 - Election of president and members of the National Assembly, comprising 109 senators and 360 members of the House of Representatives.
May 29 - Olusegun Obasanjo, current president who has already served two terms, hands over to his successor.
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.
Parties have traded accusations of plans to rig or disrupt the polls, and rights groups have accused Olusegun Obasanjo, Nigeria's president, of meddling in the process to favour the PDP.
|Women waiting to vote - many polling stations|
opened late across the country [Reuters]
Obasanjo must step down after serving for the maximum eight years and he has promised free, fair and transparent polls.
The opposition Action Congress has said thousands of supporters and several candidates have been detained.
The government ordered Nigeria's land borders to be closed all day on Saturday and Obasanjo advised Nigerians to limit travel during election days to curb violence and fraud.
On April 21, presidential elections will take place. If successfully, they will mark the first time one elected president has peacefully handed power to another since Nigeria achieved independence in 1960.