Representatives from two of Nigeria’s major political parties, including the People's Democratic Party (PDP) and the Labour Party (LP), walked out of the National Collation Center in Abuja Monday in protest against the irregularities they say were found in results announced so far.   In response, the chairman of the Independent National Electoral Commission (INEC), Mahmood Yakubu, said the early results were authenticated by electoral officials at the voting unit, dismissing claims of irregularities.