The main opposition candidates in Nigeria's presidential election have said they will not accept the results of Saturday's poll.
The presidential poll was marred by violence in several states
The Vice-President, Atiku Abubakar, and General Muhammadu Buhari, said the governing People's Democratic Party (PDP) had rigged the elections.
The PDP's Umaru Yar'Adua is reported to have a commanding lead from the results announced so far.
Foreign observers said there were serious shortcomings in the election.
Twenty-four candidates are hoping to succeed President Olusegun Obasanjo as Africa's most populous nation seeks for the first time to replace one elected civilian head with another.
Nigeria's biggest election monitoring group said the presidential poll was so flawed that it should be scrapped and held again.
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 many parts of the country elections did not start on time or did not start at all," said Transition Monitoring Group chief Innocent Chukwuma.
European Union observers have spoken of widespread abuses and violence. The Commonwealth mission said there had been some improvements compared to last week's state elections but this poll still had "serious shortcomings".
An observer for the Economic Community of West African States (Ecowas) told AFP news agency the election was "fairly acceptable" rather than "free and fair".
The Nigerian election commissioner, Maurice Iwu, said the election had been a success that would be remembered for generations.
But Mr Abubakar said the poll was one of the worst in Nigeria's history and must be held again. Voting was marred by violence and delays.
Officials had struggled to deliver some of the 60m ballot papers to stations in time for the vote.
In the central state of Nassarawa, 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 Mr 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 in the hours before polling opened on Saturday 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.
The presidential poll was running alongside elections for the National Assembly and Senate.
Election officials hope to publish detailed results by Monday night. The new government is scheduled to take power on 29 May.
Nigeria - one of the world's biggest oil producers - 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.