The electoral commission in Nigeria has postponed parliamentary elections for two days because of organisational problems.
Professor Attahiru Jega has apologised for the delay, saying ballot papers have not been delived in time to many polling stations.
But they had arrived in some of the biggest cities, such as Lagos and Kano, where voting started before the announcement of the postponement. Large turnouts were reported there.
The BBC reports the postponement is a big blow to the credibility of the electoral agency.
Professor Jega describes the situation as an emergency.
Some 73 million people have registered for the parliamentary, presidential and gubernatorial polls over the next two weeks.
Earlier, 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.
The elections will be the third nationwide polls in Nigeria since military rule ended in 1999.
Previous elections - in 2003 and 2007 - were marred by widespread ballot stuffing, voter intimidation and violence.