Hours later, gunfire rang out and subsided after about two hours in the capital of Bayelsa, an oil-rich southern state where Goodluck Jonathan, the governing party's vice-presidential candidate, serves as governor.
Jonathan was not scheduled to be in his home state until Saturday.
A local anti-government fighter said his men hoped to scuttle the elections.
Baylesa is the home of the Ijaw ethnic group, which has been at the forefront of an armed campaign against the federal government ostensibly to demand more autonomy and a greater share of oil revenues.
Elsewhere on Friday, armed men attacked a boat carrying workers to an oil rig in waters off Nigeria's unruly southern Niger Delta region, wounding six passengers, officials said.
Security forces drove off the attackers, a private security official said on condition of anonymity because of company prohibitions on dealing with the media.
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Major Sagir Musa, a military spokesman, confirmed an attack had taken place, but had no details.
A series of attacks on Western oil installations have forced Nigeria to reduce exports by a fifth for more than a year, and kidnappings of foreign oil workers in the Niger Delta are an almost weekly occurrence.
In a separate incident, a lorry-load of completed ballots was seized by soldiers in north Nigeria, according to the opposition Action Congress.
The electoral commission said the claims could not be true because new ballot papers were still arriving from abroad after a last-minute change.
In his speech on Friday, Obasanjo acknowledged that the elections for provincial governors and legislatures were flawed but asked international observers for understanding.
| Obasanjo admitted on Friday that the|
April 14 state elections were flawed [AP]
"The world is watching us and we cannot afford to disappoint ourselves, our friends and the world," he said
He appealed to election observers to "understand some of our limitations" and "not to exaggerate the negative" in Nigeria, a country which has a history of violence and electoral fraud.
Obasanjo said efforts had been made to improve on the shortcomings of past elections.