The People's Alliance for Democracy (PAD) should begin talks with the government to promote understanding, instead of only mounting attacks, Prime Minister Abhisit Vejjajiva said on Friday.
Mr Abhisit said if the government and the PAD could exchange opinions and help to each other, he believed it would be much easier to settle the ongoing disputes with Cambodia.
The PAD plans a major rally on Saturday to mount pressure on the government to do whatever it can to secure the release of Veera Somkwamkid, a Thai Patriots Network coordinator, and his secretary Ratree Pipatanapaiboon, who were sentenced to eight and six years jail respectively by the Phnom Penh Municipal Court for illegal entry and espionage.
The prime minister said while the PAD was putting pressure on the government, Cambodia had sent a letter to the World Heritage Committee asking it not to allow Thailand host a World Heritage Committee meeting, because people in Thailand had called for the government to withdraw from the World Heritage Committee.
"If we join forces, it would be easier to for us to deal with Cambodia," Mr Abhisit said.
"Cambodia has taken the opportunity given by our internal discord to tell the World Heritage Committee that Thailand should not be allowed to host next year's meeting of the committee, reasoning that the Thai people want the government to withdraw from it.
"An abrupt withdrawal from the committee would not be good for our efforts to protect our interests," Mr Abhisit said.
The prime minister insisted it is necessary for the government to continue its membership of the World Heritage Committee. Thailand should not admit defeat or make an abrupt withdrawal from the committee, he added.
Mr Abhisit said the government's attempts to reach an understanding with the PAD were going in a favourable direction, but declined not to go in details.
On the PAD's planned rally on Saturday, the prime minister said everyone should bear in mind that if the situation developed in an unfavourable way it would not be good for the country.
He reaffirmed that the PAD protesters would not be allow to intrude into the Government House compound.
Police would continue to talk the PAD leaders and ask them to reopen part of Ratchadamnoen Nok Avenue to traffic.