Nigeria's biggest election monitoring group has said Saturday's presidential poll was so flawed that it should be scrapped and held again.
Police fired on rioting crowds in the northern state of Katsina
The Transition Monitoring Group said a winner could not be announced on the basis of results from half the country.
Voting was marred by violence in which several people died. There was also an attempt to blow up the election HQ.
Voting dragged into a second day in four states where ballot papers arrived late, officials said.
The Independent National Electoral Commission (Inec) had earlier said that voting would not be extended into Sunday, despite the fact that ballot delivery failures delayed the poll and some ballot boxes were stolen.
"In many parts of the country elections did not start on time or did not start at all," Transition Monitoring Group chief, Innocent Chukwuma, told Reuters news agency.
He said the official electoral commission had not been properly prepared for the vote.
The head of the European Union monitoring mission, Max van den Berg, said he feared there had been no improvement on last week's regional polls, which were marred by 50 deaths and widespread fraud.
Twenty-four candidates are seeking to replace outgoing President Obasanjo in Africa's most populous nation.
The presidential poll was running alongside elections for the National Assembly and Senate.
Election officials hope to publish results by Monday night.
The new government is scheduled to take power on 29 May.
Officials had struggled to deliver some of the 60m ballot papers to stations in time for the vote.
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
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 policemen 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.
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.