By Thursday morning reports said more than 800 Cambodian soldiers armed with assault rifles and rocket launchers had been deployed to the temple, while some 400 Thai soldiers had been stationed nearby.
Al Jazeera's correspondent Marga Ortigas, reporting from the Thai side of the border, said road blocks have been put up some 10 kilometres from the temple.
She said Thai protesters, who claim the temple belongs to Thailand, have been gathering near the road blocks, but that many locals feared the protesters were actually working against peace.
The confrontation comes after Unesco, the UN's cultural body, declared the 11th century temple a World Heritage site last week.
In 1962, the International Court of Justice awarded the Preah Vihear ruins and the land it occupies to Cambodia, a decision that still rankles many Thais.
|Preah Vihear temple has been designated a Unesco World Heritage site [EPA]
Thai activists and some government officials fear the temple's new status will jeopardise their country's claims to land adjacent to the site.
Brigadier Chea Keo, Cambodia's top army commander at Preah Vihear, said the situation could worsen if the Thai forces continued to swell their ranks.
"If the Thais keep adding more troops the situation will escalate, but we try to be patient," he said on Thursday.
"They want us to do something first but we try to remain calm."
Cambodian officials claim Thai soldiers began the crossing the border on Tuesday after three Thai protesters were arrested for jumping an immigration checkpoint to reach the temple. They were returned to Thailand on the same day.
Thailand however denied the trespass and insisted that soldiers were patrolling its side of the border.
The border area of 4.6 square km has been at the centre of a long-running territorial dispute as the temple's main compound lies inside Cambodia but its most accessible entrance is at the foot of a mountain in Thailand.
The temple dispute has also led to heightened political tensions in both countries.
Cambodia is preparing for general elections on July 27 during which Hun Sen, the country's prime minister, is expected to extend his grip on power after 10 years iin office.
In Thailand, there have been weeks of streets protests and severe criticism of the government led by Samak Sundaravej, the prime minister, for supporting Cambodia's Unesco bid.