A voter casts his ballot at a polling station in Abuja, Nigeria

Nigeria's main opposition party says it will go to court to challenge the official results of last Saturday's presidential election.

Speaking to reporters Friday, the chairman of the All Nigeria People's Party said his candidate won the election, but that officials gave victory to ruling party candidate Umaru Yar'Adua.

Opposition candidate Muhammadu Buhari previously ruled out going to court over the dispute. 

The official results showed Buhari finishing a distant second to Yar'Adua in the April 21 poll.  But election monitors say the voting process was badly marred by cases of ballot box-stuffing, voter intimidation and polls failing to open.

The All Nigeria People's Party says it will not recognize or cooperate with any government formed as a result of last week's vote.

On Wednesday, a coalition of 20 opposition parties called for new elections and the dissolution of Nigeria's electoral commission. 

The parties have scheduled protests on May 1 to demand the presidential vote be annulled.