The result of the Senate election will be announced as planned, although the Election Commission (EC) has received more than 300 complaints, including 39 fraud-related cases to investigate.
EC secretary-general Sawang Boonmee said there are 333 complaints, 90 of which involve corruption, and it will take some time to investigate them.
Most of the complaints involve candidates with questionable backgrounds applying to stand in professional groups for which they were not eligible.
He said the EC had instructed election officials, including police, to monitor activities that could be regarded as election fraud before tomorrow's final round of voting.
According to the EC secretary-general, it is against the law for politicians or political parties to assist senatorial candidates in elections in any way.
However, he admitted that political groups have tried interfering in the Senate election, and it is difficult to prevent this when their interests are aligned with those of the candidates.
He said the candidates can protest during the voting process and petition the court after the election. However, he asked them not to disrupt or stall the Senate poll.
"The election officials must strictly follow the procedures and timeframe to minimise legal disputes that could nullify the result," he said.
Meanwhile, a source close to the EC said the commission has identified four possible forms of irregularities ahead of tomorrow's voting in which 3,000 candidates will compete for the 200 upper-chamber seats.
The first involves candidates offering to sell their votes in exchange for a six-digit amount of money or for a position on the new senator's team.
The second involves attempts by political groups to persuade candidates to join their network.
The third involves outside parties booking hotel rooms for the candidates so they can meet ahead of the election day and see how many votes they have. This group plans to sell their votes for a seven-digit amount.
The last involves the interference of political officeholders who approach candidates to join the groups they support so they can be elected.
"These activities have all been reported in several areas, especially in the North. Investigators are collecting evidence of possible wrongdoing," said the sources.
Caretaker Senator Somchai Swangkarn wrote on Facebook on Monday that political parties from the government and opposition camps are competing for control of the Senate by having their affiliates run in the contest.
He said parties want control of the Senate to drive their political agenda, which includes passing key bills such as the referendum amendment, the charter amendment and amnesty for political offenders.