Nigeria has postponed its parliamentary election until next Saturday - the second such delay in two days. The vote was initially due to take place on Saturday, but staff and papers failed to materialise at polling stations around the country. After first calling the election off until Monday, officials further delayed it until Saturday. The election commission's decision means presidential and state elections have also been pushed back. The BBC's Caroline Duffield in Lagos says the country's political culture of vote-rigging and violence has made it difficult for people to accept the official explanations for the delay. She says many voters - and some politicians - think political interference caused Saturday's chaos. The elections are seen as a vital test of Nigeria's democratic credentials. Electoral chief Attahiru Jega was brought in last year to overhaul a system often regarded as flawed. The electoral chaos has led some to question his suitability for the job. Announcing the second delay, Mr Jega said the decision had the backing of all political parties. "Requests to reschedule the national assembly elections have come from a cross section of stakeholders, including political parties and civil society organisations," he said. "We are more determined now to ensure that the 2011 elections are free, fair and credible." The People's Democratic Party (PDP) has won all three elections since the end of military rule in 1999, amid widespread claims of rigging and cheating. Nigerian elections are also marked by violence - and security had been high in the run-up to Saturday's aborted poll. But Amnesty International said at least 20 people had been killed in election-related violence over the last two weeks. The voting process had already started on Saturday, with large turnouts reported in cities such as Lagos and Kano, before Mr Jega announced the initial postponement. Some 73 million people have registered for the election, where they will vote for 360 seats in the House of Representatives, and 109 in the Senate. The PDP holds more than three-quarters of the seats in both houses.