Parliamentary elections in Nigeria have been postponed until Monday because of organisational problems, officials say.
The electoral officials - who have apologised for the delay - say ballot papers have not been delivered in time to many polling stations.
The decision is seen as a big blow to the credibility of the electoral body in Africa's most populous country.
Some 73m people have registered for the parliamentary, presidential and gubernatorial polls over two weeks.
Security has been high, with borders closed and only election officials, security forces and emergency personnel allowed to drive on roads during voting.
Earlier, politicians were urged to put a stop to campaign violence.
Amnesty International said at least 20 people had been killed in related attacks and clashes over the last two weeks.
A bomb was thrown into a police station in the city of Bauchi on Friday in an apparent attempt to cause panic. No casualties were reported.
Police in the Niger Delta also said they had arrested two men driving a minibus filled with assault rifles, ammunition and a rocket launcher.
"In order to maintain the integrity of the elections and retain effective overall control of the process, the commission has taken the very difficult but necessary decision to postpone the national assembly elections to Monday," Independent National Electoral Commission (Inec) head Attahiru Jega said on Saturday.
"It is an emergency," he added.
It is understood that aircraft were supposed to be flying in ballot papers and accreditation details from overseas but those planes were diverted away from Nigerian airspace, the BBC's Caroline Duffield in Lagos reports.
There were angry scenes in polling stations across the country as word began spreading that the elections were postponed, our correspondent says.
She adds that the move raises fears among some that Mr Jega's grip on his staff at the election commission is not tight enough, and people will wonder whether the coming elections over the next two weeks will run smoothly.
The voting process had already started with large turnouts reported in cities such as Lagos and Kano before the announcement by Mr Jega.
The elections will be the third nationwide polls in Nigeria since military rule ended in 1999.
The previous votes - in 2003 and 2007 - were marred by allegations of widespread ballot stuffing, voter intimidation and violence.
Security forces were also accused of siding with the People's Democratic Party (PDP), which has dominated politics since the return to civilian rule.
Mr Jega earlier told the BBC that if Nigerians wanted to peacefully defend their votes at the polling stations, that was their right.
And he threatened sanctions against any political leader engaging in violence or vote-rigging, even warning he would resign if necessary.
In the election, 360 seats in the lower house of parliament, the House of Representatives, and 109 in the upper house, the Senate, will be contested. The PDP holds more than three quarters of the seats in both houses.
To win at the first round, a candidate not only needs the majority of votes cast, but at least 25% of the vote in two-thirds of Nigeria's 36 states. Goodluck Jonathan, of the PDP, reached that threshold in 31 states; runner-up Muhammadu Buhari of the CPC only did so in 16 states.
Nigeria's 160 million people are divided between numerous ethno-linguistic groups and also along religious lines. Broadly, the Hausa-Fulani people based in the north are mostly Muslims. The Yorubas of the south-west are divided between Muslims and Christians, while the Igbos of the south-east and neighbouring groups are mostly Christian or animist. The Middle Belt is home to hundreds of groups with different beliefs, and around Jos there are frequent clashes between Hausa-speaking Muslims and Christian members of the Berom community.
Despite its vast resources, Nigeria ranks among the most unequal countries in the world, according to the UN. The poverty in the north is in stark contrast to the more developed southern states. While in the oil-rich south-east, the residents of Delta and Akwa Ibom complain that all the wealth they generate flows up the pipeline to Abuja and Lagos.
Southern residents tend to have better access to healthcare, as reflected by the greater uptake of vaccines for polio, tuberculosis, tetanus and diphtheria. Some northern groups have in the past boycotted immunisation programmes, saying they are a Western plot to make Muslim women infertile. This led to a recurrence of polio, but the vaccinations have now resumed.
Female literacy is seen as the key to raising living standards for the next generation. For example, a newborn child is far likelier to survive if its mother is well-educated. In Nigeria we see a stark contrast between the mainly Muslim north and the Christian and animist south. In some northern states less than 5% of women can read and write, whereas in some Igbo areas more than 90% are literate.
Nigeria is Africa's biggest oil producer and among the biggest in the world but most of its people subsist on less than $2 a day. The oil is produced in the south-east and some militant groups there want to keep a greater share of the wealth which comes from under their feet. Attacks by militants on oil installations led to a sharp fall in Nigeria's output during the last decade. But in 2010, a government amnesty led thousands of fighters to lay down their weapons.