It would be "inappropriate" to allow outside involvement in the poll, which is due in June or July and comes after deadly street protests last year, said Suthep Thaugsuban.
"I don't respect 'farangs'. We do not have to surrender to them," he said, using the Thai word for "Westerners".
Anti-government "Red Shirts" have recently called for international observers to be present.
"I am surprised that Red Shirts do not respect our country's sovereignty... I don't understand why they constantly call for foreign involvement, it is inappropriate," Suthep told reporters.
Thailand has never officially allowed monitors to operate in polling stations during its elections, but the Asian Network for Free Elections (ANFREL) has been allowed to work in the country during the last two votes.
"It is true they do not allow the monitors to enter polling stations but we observe from outside, it's quite easy," said executive director Somsri Hananuntasuk.
She said she was "surprised" at Suthep's comment, adding that other members of the ruling Democrat party, including Prime Minister Abhisit Vejjajiva "welcomes international observers".
Thailand has suffered years of political instability, punctuated by unrest and military intervention, seeing 18 actual or attempted coups since 1932 when the country became a constitutional monarchy.
Commentators believe the upcoming poll is likely to highlight the country's divisions.
The elected Prime Minister Thaksin Shinawatra, a hero to many in the Red Shirt movement, was ousted in a coup in 2006, sparking ongoing political turmoil.
Abhisit's government is seen as undemocratic by the Reds as it came to power in an army-backed 2008 parliamentary vote after a court ruling threw out the previous administration, made up of Thaksin allies who had won an election.
Thailand, known as the "Land of Smiles", is usually regarded as friendly to foreigners.
According to Abhisit the kingdom received 15.8 million overseas tourists last year and the sector generated 585 billion baht ($19 billion), or 7 percent of economic output.
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