Thailand's main opposition party says it has submitted evidence to the electoral commission that the ruling party has committed electoral fraud.
The Thai leader is campaigning hard ahead of the polls
The Democrats allege ruling party officials were involved in a plan to hire candidates to stand for small opposition parties in a 2 April poll.
As the Democrats are boycotting it, it would lend it more legitimacy.
But Prime Minister Thaksin Shinawatra, who called the poll in a bid to end protests, dismissed the charges.
Thousands of people are camped on the streets outside Government House demanding his resignation, accusing him of misrule and corruption.
The opposition's planned boycott of the upcoming election leaves ruling party candidates unchallenged in some seats.
This is potentially problematic for Mr Thaksin's Thai Rak Thai party because Thailand's election laws say that a candidate standing unchallenged must win at least 20% of eligible votes in the district.
That might be hard to achieve in some parts of the country where the ruling party is not that popular, the BBC's Chris Hogg says.
Protests against Mr Thaksin are continuing in Bangkok
On Sunday, the opposition Democrat Party said ruling party officials were involved in a plan to hire people to stand as candidates for small opposition parties in the upcoming polls.
Election Commission chief Vasana Puemlarp said he was ready to examine the opposition's claims, which the party said it would submit evidence on Monday.
The Democrats pledged to expose more wrongdoing if the Thai leader failed to address the allegations.
Mr Thaksin called the charges "nonsense" and said his party had done nothing wrong.
Meanwhile, representatives of Mr Thaksin's party, the main opposition parties and the organisers of the mass street protests held a three-hour meeting on Monday to discuss talks between the premier and his opponents, but failed to reach an agreement, the AFP news agency reported.
The opposition and protest organisers were seeking a televised debate with Mr Thaksin, but the ruling party proposed closed-door talks and said that the opposition had to respect the result of the upcoming polls.
Mr Thaksin, on the campaign trail, called 2 April a "judgement day" for Thai politics. "I'm asking all the people to go to vote, to clearly show the real decision of the Thai people," he said in a speech.
"All the parties involved, including those boycotting the election, must accept that decision."