With calls for Nigeria's presidential and parliamentary polls to be cancelled, the BBC looks at what the initial observations are from the different teams monitoring the polls.
Transition Monitoring Group
The Transition Monitoring Group (TMG), with 50,000 Nigeria observers on the ground, called for the polls to be cancelled.
Many observers said the polls were below acceptable standards
TMG head Innocent Chukwuma said the irregularities were so numerous and so far-reaching that "the election was a charade and did not meet the standards required for democratic elections".
"Our monitors throughout the country... documented numerous lapses, irregularities and electoral malpractices that characterised the election in many states," he said.
"We therefore reject it and call for its cancellation," he said, calling for a re-run within three months.
The electoral commission (Inec) "failed wilfully in their responsibility to conduct free, fair and credible elections", he said.
"We call on the international community not to recognise these discredited elections and not to confer legitimacy on any government that emerges therefrom."
The European Union, who had 150 observers monitoring Nigeria's presidential and parliamentary elections, said the polls "have fallen far short of basic international and regional standards for democratic elections and... cannot be considered to have been credible."
In a statement, they also said there were credible reports indicating at least 200 were killed - including voters, police and candidates - in both the state and national elections.
The head of the European Union monitoring team, Max van den Berg, told the BBC it was one of the worst elections that the EU had observed.
"I can compare it of course with 2003, when I was the chief observer, and I had expected, really, after that was a very disappointing election, that we would see now something better. But we have not seen that, and the credibility is not there.
"The whole thing was not at all living up to the hopes of the Nigerian people, chaotic, and I would say it left them behind, demoralised."
"EU observers witnessed examples of ballot box stuffing, alteration of official result forms, stealing of sensitive polling materials, vote buying and underage voting," he said.
Commonwealth observer group
The Commonwealth group of 17 observers talked of significant shortcomings but said they had been an improvement on the state polls a week before.
"The deferred polling hours posed a challenge... in so far as the count was concerned. Darkness fell while counting was still in progress and no provision had been made to provide artificial lighting for such a contingency," the statement continued.
Team leader Joseph Warioba said contestants generally enjoyed freedom of assembly, movement and expression.
"We got reports that in several places the delay was there. The polls were supposed to open at 10 o'clock but we got reports in certain places they opened at one, at three and some at five or later.
"What we have received is not widespread. Stuffing [of ballot boxes] was reported in one area, pre-marked papers in one area. So we cannot take that as general, but it is a shortcoming. I'm waiting until I have all the facts before I make an assessment."
International Republican Institute
The US-based International Republican Institute with 59 observers said the entire electoral process had failed to meet international standards.
"Nigeria's election process, which we recognise is still continuing and thus far incomplete, falls below the standards which Nigeria itself has set in previous elections and also falls below international standards, witnessed by IRI and members of this delegation throughout the world, " said the IRI.
"Neither the spirit of Nigerians who went to the polls to cast their ballots nor the dedication of the thousands of poll workers struggling to execute their responsibilities in polling stations throughout the country were matched by their leaders."
It said it observed "underage voting, voter registration list errors, stuffed ballot boxes, group voting, party observers and police instructing individuals on who to vote for, lack of privacy for voting, lack of results sheets and other materials, falsified results sheets, and early closings".
Joint Action Forum
The local Nigerian Joint Action Forum, comprising 50 civil society organisations, wants a re-run of the elections.
"An appraisal of the electoral process reveals an Inec that is not only partisan in siding with the ruling PDP but is generally unprepared for the elections.
"Worst of all is the presidential poll held yesterday that is marred by voters' apathy as a result of Inec failings to hold as scheduled the Presidential and National Assembly elections in some parts of the country. States such Anambra, Imo and Plateau dispatched election materials at about 6pm when voting ought to have been concluded an hour earlier."
Economic Community of West African States (Ecowas)
An Ecowas observer described the polls as "fairly acceptable" rather than "free and fair".
"National Assembly elections didn't hold in some constituencies in Lagos, Ogun, Oyo, Rivers and several other places because of lack of ballots while in Kano, presidential ballot was missing.
"Despite all the failings by Inec at all stages of the electoral process, we commend the resilience and commitment of Nigerians to vote and ensure that their votes count."
National Democratic Institute (NDI)
With 61 monitors deployed in 14 states, the National Democratic Institute says the "electoral process has failed the Nigerian people".
"In many places and in a number of ways, the electoral process failed the Nigerian people. The cumulative effect of the serious problems the delegation witnessed substantially compromised the integrity of the electoral process. As a result, at this stage, it is unclear whether the April 21 elections reflect the will of the Nigerian people.
"A major problem that marred this stage of the electoral process was that polling stations in many states opened hours late, closed early or failed to open at all. This represented a fundamental barrier to popular political participation and most likely disenfranchised many prospective voters.
"Regrettably, the 2007 polls represent a step backward in the conduct of elections in Nigeria. At the same time, there are positive trends in the country's democratisation process that give rise to hope.
"The serious flaws witnessed during this electoral process threaten to further erode citizen confidence in the country's democratic institutions."