Violence and widespread disruption have marred presidential and parliamentary elections being held in Nigeria.
Police fired on rioting crowds in the northern state of Katsina
Many of the 120,000 polling stations failed to open for hours and polling was extended in many places beyond the official closing time.
There were a number of killings, thefts of ballot boxes and an attempt to blow up the election headquarters.
Twenty-four candidates are seeking to replace outgoing President Obasanjo in Africa's most populous nation.
The presidential poll is running alongside elections for the National Assembly and Senate.
European Union election observer Max van den Berg said he feared there may not be an improvement on last week's regional polls, which were marred by 50 deaths and widespread fraud.
However, the chairman of the Independent National Electoral Commission, Professor Maurice Iwu, said that is was a case of "so far so good".
"We are pleased with the outcome of the polls. Taking in the big picture, Nigerians really demonstrated their commitment to democracy," Professor Iwu said.
All polling stations have now closed but the BBC's David Bamford in Abuja says tension remains high.
Election officials hope to publish results by late on Monday.
The new government is scheduled to take power on 29 May.
The officials had struggled to deliver some of the 60m ballot papers to stations ahead of opening time.
60m registered voters
120,000 ballot boxes
360 House of Representative seats to be elected
109 Senate seats to be elected
24 presidential candidates
Main contenders: Atiku Abubakar for the AC, 60-years-oldMuhammadu Buhari, ANPP, 64Umaru Musa Yar'Adua, PDP, 55
Polls open 1000-1700 (local time) (0900-1600 GMT)
To avoid a run-off, a candidate needs highest number of votes overall and at least 25% of votes in 24 of the 36 states
In the central state of Nassarawa, close to the capital, Abuja, a number of policeman were killed while escorting election officials with the papers.
Police fired on crowds in the northern state of Katsina, the home state of both the governing party candidate, Umaru Musa Yar'Adua and one of his main opponents, Muhammadu Buhari.
Four people were killed in clashes there after only half the voting papers arrived.
In nearby Kano, men armed with cutlasses and guns stole ballot boxes while in Onde state, in the south-west, men disguised as policemen abducted election officials.
The boldest attempt to disrupt polling was on Friday when a petrol tanker laden with gas cylinders was used in an attack on the electoral commission's headquarters in Abuja.
The attackers tried to roll the unmanned tanker into the building, but the vehicle missed its target and came to a halt.
President Obasanjo said despite flaws, Nigeria could achieve a peaceful and democratic handover of power.
Speaking after voting, he said that "those who do not want these elections to take place, they have left no stone unturned to make sure it does not hold".
Mr Buhari said no-one should claim victory because there were so many irregularities in polling.
He said he would call his supporters out on to the streets if the governing Peoples Democratic Party (PDP) declared victory.
Rival opposition candidate Atiku Abubakar described the election as a "national tragedy".
His Action Congress party accused the authorities of ensuring "inadequate supply of voting materials" in its strongholds.
Our correspondent says the comments show Nigeria could be in for some political turbulence as it seeks for the first time to replace one elected civilian head of state with another.
Nigeria is one of the world's biggest oil producers. It is of key strategic interest to both the West and the growing economies of the East.
But despite the country's huge oil wealth, tens of millions live in poverty.
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