"Hundreds of thousands of people were not included on voter lists," said Zharmakhan Tuyakbai, the leader of For a Fair Kazakhstan.
He also said people were nearly forced to vote in an electronic format at many polling stations.
Tuyakbai said his party would do all in its legal power to make justice prevail and cancel the election results that he said were falsified.
"We do not want any bloodshed," he said.
Earlier, Tuyakbai promised that the opposition would not organize any mass protests if it lost the elections.
"If the elections are fair and Nazarbayev wins, we will not stage any protests and congratulate him on his triumph," Tuyakbai said.
CIS observers said they had not registered any violations.
Vladimir Rushailo, the executive secretary of the CIS and head of the CIS monitoring mission to the Kazakh elections, said the "Saylau" (election) electronic voting system met modern standards of information technology and reflected the global trend of paperless voting.
