'Splinter group' blamed
Wayne Hay, Al Jazeera's correspondent in the region, said about 4,000 protesters had descended on the disputed border with Cambodia "to try to force some Cambodian villagers out of the area and back into Cambodia proper".
"There is a Thai police blockade that the PAD broke through with a bit of violence and also about four or five hundred villagers came out to try to stop the PAD from progressing," Hay said.
"There are also some skirmishes. We are hearing that there were several injuries as [protesters] went through that police blockade."
The PAD's leaders in Bangkok had distanced themselves from the protest, saying that a "splinter group" was behind it, Hay said.
Tear gas deployed
Abhisit Vejjajiva, the prime minister, 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," he told reporters in Bangkok.
The temple, granted UN World Heritage status in July 2008, has been the scene of several deadly battles between Thai and Cambodian troops over the past year.
Cambodia said on Thursday it had deployed riot police with dogs, batons and tear gas at the temple.
Deaths in gunbattle
The Yellow Shirts have demanded that the government push Cambodian forces out of the disputed area around it.
The two countries have been at loggerheads for decades over Preah Vihear.
The World Court ruled in 1962 that Preah Vihear belonged to Cambodia.
But Thailand retains rights to enter the ancient Khmer temple, which has crumbling stone staircases and elegant carvings.
A gunbattle in the temple area in April left three people dead while four people died in clashes in 2008.