Pheu Thai Party secretary-general Phumtham Wechayachai on Monday demanded the Election Commission reveal its formula for deciding on the distribution of party-list MPs, to clear up doubts about the process.
However, Deputy Prime Minister Prawit Wongsuwan said the EC was doing its job well.
"The EC seems to have no criterion for calculating the numbers of list MPs. The calculation changes daily, at whim," Mr Phumtham said.
If the EC was unable to clarify its calculation method, there could also be doubts when it decides to give out yellow or red cards to candidates, he said.
Without clarification, the EC could be suspected of being the tool of some party, Mr Phumtham said.
Gen Prawit said the EC was complying with regulations and working well, and mass media should help send the message to the people demonstrating against the EC.
He denied he was coordinating the formation of the next government. He also said that authorities were ready to maintain order if there were protests.