Thai "Yellow Shirt" protesters clashed with police and villagers on Saturday near an ancient temple on the Cambodian border at the centre of a dispute between the two countries, television showed.
Demonstrators broke through barricades and were moving towards the 11th century Preah Vihear temple, the scene of several deadly battles between Thai and Cambodian troops over the past year, the footage showed.
The protest by the royalist movement, known as the People's Alliance for Democracy (PAD), came as rival "Red Shirts" massed in Bangkok to mark the third anniversary of a coup that toppled then-premier Thaksin Shinawatra.
Television channels showed yellow-clad protesters armed with sticks trying to beat local villagers and Thai riot police, who pushed back with shields.
Current Prime Minister Abhisit Vejjajiva said security forces were trying to persuade the protesters to back down, amid fears that their actions could spark further conflict with Cambodia.
"I am asking the police and soldiers to negotiate with the PAD," Abhisit told reporters in Bangkok.
The Yellow Shirts have demanded that the government push Cambodian forces out of the disputed area around the temple, where tensions have been high since the ruins were granted UN World Heritage status in July 2008.
Cambodia said on Thursday it had deployed riot police with dogs, batons and tear gas at the temple.
The two countries have been at loggerheads for decades over Preah Vihear.
The World Court ruled in 1962 that it belonged to Cambodia, but the most accessible entrance to the ancient Khmer temple with its crumbling stone staircases and elegant carvings is in northeastern Thailand.
The last gunbattle in the temple area in April left three people dead while clashes there in 2008 killed another four people.
The Yellow Shirts helped topple Thaksin in 2006 and then blockaded Bangkok's airports in December to bring down the previous, pro-Thaksin government, but have recently turned their fire on Abhisit's administration.