The group issued an ultimatum threatening to step up their rally in the capital after Veera Somkwamkid -- a former leader of the Yellow movement -- and his secretary were sentenced by a Cambodian court on Tuesday.

"The Thai government has three days to comply with our demands, including bringing back those two people from Cambodia," said spokesman Parnthep Pourpongpan.

Hundreds of Yellow Shirts have camped out around the government's compound since last week, demonstrating against its handling of a border dispute with neighbouring Cambodia, and the group plans a larger rally on Saturday.

Despite relatively small numbers compared to their arch enemies -- the anti-government "Red Shirts" whose most recent rally attracted nearly 30,000 people -- protesters have managed to choke off streets around Government House.

Veera and Ratree Pipattanapaibul were were found guilty of illegal entry into Cambodia, trespassing on a military area and spying.

The pair were among seven Thais arrested in Cambodia on December 29 while they were inspecting disputed border territory.

The other five -- including a ruling party politician -- received suspended sentences for illegal entry on January 21 and have since returned to Thailand.

The Thailand-Cambodia border has never been fully demarcated, partly because it is littered with landmines left over from decades of war in Cambodia.

Thai Deputy Prime Minister Suthep Thaugsuban said he was "disappointed" with the verdict.

He warned protesters to remain calm and said security had been stepped up around key government buildings and the Cambodian embassy in Bangkok.

"Do not let this verdict trigger violence, we must solve the issue in peaceful way," he said.

Yellow Shirts are a force to be reckoned with in Thailand's colour-coded politics and have helped to claim the scalps of three governments in under five years, including that of fugitive former premier Thaksin Shinawatra.

Officially known as the People's Alliance for Democracy, they seized two Bangkok airports in late 2008, leaving more than 300,000 travellers stranded.