Nigeria's election commission on Saturday rejected claims of political interference after its last-minute postponement of presidential elections, as voters caught unawares hit out at the announcement. The Independent Electoral Commission (INEC) announced a one-week delay just hours before polls to elect the head of Africa's most populous nation were due to open at 0700 GMT. It blamed logistical difficulties, including problems in the distribution of ballot papers and results sheets, as well as sabotage, after three fires at its offices in two weeks. But the two main political parties claimed the delay was part of a conspiracy to rig the results. International observers called for calm. President Muhammadu Buhari, 76, and his main challenger Atiku Abubakar, 72, returned to Abuja from their home towns in the northern states of Katsina and Adamawa. In the capital, INEC chairman Mahmood Yakubu, rejected assertions of a political conspiracy. "Our decision was entirely taken by the commission," he told a news conference. He added that it had "nothing to do with political influence". Voters arrived early to vote only to find many of the nearly 120,000 polling units deserted and unstaffed. "Why didn't they announce the delay earlier? Why make the announcement in the middle of the night?" asked Chidi Nwakuna, a businessman in the southern city of Port Harcourt. Just hours before the announcement, Boko Haram jihadists killed at least eight people in an attack on the northeastern city of Maiduguri. - Logistical problems - Rumours began circulating late on Friday about a possible postponement after widespread reports of problems with the delivery of election materials, including ballot papers. INEC commissioners held emergency talks and after examining the logistics plans concluded the timetable was "no longer feasible", Yakubu said in his early hours announcement. Presidential and parliamentary elections are now set for February 23, and governorship and state assembly elections have been pushed back to March 9. "This was a difficult decision for the commission to take but necessary for the successful delivery of elections and the consolidation of our democracy," he added. The two main political parties swiftly accused each other of orchestrating the delay as a way of manipulating the vote, a sentiment echoed by voters, some of whom had travelled long distances to vote in their hometowns. "I see this postponement of the election as a... ploy to rig," said Oyi Adamezie in Warri, in the southern state of Delta. - Disappointment - Nigeria has postponed voting before: in 2015, INEC announced a six-week delay just one week before the election, citing security concerns linked to Boko Haram. The six-week delay was seen as a way for then president Goodluck Jonathan to claw back votes after a strong challenge from Buhari, an opposition candidate. The same argument may be made again, with little to separate Buhari and Abubakar in the campaign. Yet even before the delay announcement, challenges were apparent in the vote's organisation. In many areas suffering intermittent electricity supply and poor road infrastructure, thousands of INEC agents had been working into the night to deliver election materials. "They had much time to prepare," said Austin Onwusoanya, a civil servant in the largest city Lagos who was to manage a polling unit that now stood unused. "There are other things going on." - Campaign trail - The postponement comes after an election campaign in which Buhari had sought to portray himself as a continuity candidate. He came to power in 2015 on a promise to defeat Boko Haram, tackle rampant corruption and improve the country's oil-dependent economy. But the jihadists still mount attacks, and there is growing insecurity elsewhere. Slow growth, as the economy limps back from recession, has also hit Buhari's stock. So, too, has the perception he has only targeted political opponents as part of his high-profile anti-corruption campaign. Buhari's alleged shortcomings have been a feature of Abubakar's campaign, who has billed himself as a modern, energetic and pro-business leader. The former vice-president's past has also featured prominently, as the ruling party resurrected controversies from his time in office and alleged links to corruption. The election -- the sixth in the 20 years since civilian rule was restored -- is likely to be one of the last times men of Buhari and Abubakar's generation will feature so prominently. They have been fixtures on Nigeria's turbulent political scene for decades and are the oldest on the ballot. Just over half of the 84 million registered voters are aged 18-35, prompting calls for more representative candidates unburdened by involvement in Nigeria's traumatic past.