Lagos, Nigeria (CNN) -- Nigeria postponed its parliamentary election Saturday due to a "terrible unfortunate emergency" caused by the late arrival of voting materials, the head of the election commission said.
Nigerians will instead go to the polls Monday, said Attahiru Jega, chairman of the Independent National Election Commission.
"We cannot proceed with these elections if we want them to be free, fair and credible if there are no result sheets," Jega said. "We cannot bury our heads and say there are no problems. It is regrettable. It is unfortunate. It should not have happened."
Jega said he takes full responsibility for the fiasco but he said a vendor that was supplying results sheets and ballots was unable to deliver them on time. The vendor, said Jega, cited the diversion of planes to carry relief supplies to earthquake-stricken Japan as the reason for the delay.
Citizens of Africa's most populous nation were supposed to vote Saturday for 360 House of Representatives seats and 109 Senate seats. They are scheduled to vote next Saturday in a presidential election and for state governors the following week.
Before the logistical problems, the election, the most expensive in Nigeria's history, had already been marred by riots, bombings and assassinations.
"The unprecedented levels of violence that have seen several people either killed, maimed, kidnapped or intimidated for political reasons pose the single most significant threat to the conduct of general elections," warned the Nigeria Elections Situation Room -- a forum of groups focusing on the upcoming elections.
Human Rights Watch estimates at least 70 people have been killed in political violence in the run-up to the voting.
The European Union described Nigeria's 2007 elections as the worst they had ever seen anywhere in the world, with rampant vote rigging, violence, theft of ballot boxes and intimidation. Nigeria had hoped to gain a cleaner image this year.
CNN's Christian Purefoy contributed to this report.