Parti Keadilan Rakyat (Keadilan) vice-president Tian Chua claimed that he has received a report on abuses in the manner the election was conducted.
He accused the Election Commission of fraudulent activities and added that it was not being fair.
According to Bernama, he said the decision to extend the voting time for the seats in Selangor "was a violation of the Election Act".
Asked if the party would reject the outcome even if it were to win seats, he said no decision has been made.
"We will discuss this and furnish strong evidence to back our allegations," he added.
Unofficial result indicates that Tian Chua has lost to Gerakan's Ng Lip Yong by 10,000 votes in the Batu parliamentary seat.