Citing intelligence reports, Pol Maj Gen Piya said some groups intend to use violence and burn government office buildings. They are also planning to capture senior officials, such as the provincial governor and district office chief, he said.
Police are keeping a close watch on those groups, Pol Maj Gen Piya said, adding that Royal Thai Police chief Adul Saengsingkaew had stressed the need for riot police to comply with international crowd control principles in handling the protest.
Riot officers must try to keep a distance between themselves and the protesters to avoid clashes, he said.
But if the protesters use violence which endangers the public, the authorities must step up the scale of crowd control measures according to the extent of the threat.
Riot police must comply with the law by warning the protesters of crowd control measures, such as the use of tear gas.
There must also be a senior official responsible for directing crowd control operations and who is accountable for the decisions, Pol Maj Gen Piya said.
He said police had to use tear gas to disperse the protesters who blockaded the Khuan Nong Hong intersection in Cha-uat district on Monday, as they refused to move despite being issued with a tear gas warning.
The protesters threw acid at police so the police had to use tear gas to control the situation. Some 78 police officers were injured in the clashes and about 10 police vehicles were set alight.
Nakhon Si Thammarat governor Wiroj Jiwarangsan said Tuesday that the public disaster prevention law, which has been invoked to handle the protesters, will be extended for another 24 hours.
Under the law, the governor has declared Khuan Nong Hong intersection off-limits to the public.
The law is intended to keep members of the public away from the protest site for their own safety. It was not a measure to limit their freedom of movement, Mr Wiroj said.
A source at the cabinet meeting Tuesday said Prime Minister Yingluck Shinawatra was worried the protest could adversely affect foreign confidence in the country's economy.
The source added that Ms Yingluck had rejected a suggestion made during the course of the cabinet meeting that the Democrat Party be invited to advise the government on how to best solve the rubber farmers' problems.
Ms Yingluck said after the cabinet meeting Tuesday that the protests were a local issue and the governor and authorities should still be able to deal with them under the public disaster prevention law.
Deputy secretary-general to the prime minister Thawat Boonfueng said the government will not yet resort to imposing a harsher security law, such as the Internal Security Act (ISA), in the protest area.
The government has authorised Mr Wiroj and police to control the situation.
The ISA will not be invoked unless local authorities recommend it be implemented to the government.
The blockade at the Khuan Nong Hong intersection on Highway No.41 continued Tuesday.
The situation was generally calm. Many objects, including concrete pipes, tree branches and tables were still obstructing the intersection.
About 1pm, the protesters cut down a tree and placed it across the road to prevent police from entering the protest site.
Police set up checkpoints on Highway 41 to search vehicles to prevent dangerous objects being supplied to the protesters.
They also advised motorists to take alternative routes and avoid Khuan Nong Hong intersection.
National police chief Pol Gen Adul said police have issued warrants for the arrest of 19 protesters. Some of them have already been detained.
Parik Panchuay, a coordinator of rubber farmers in Cha-uat district, said Tuesday the government should meet for talks with representatives of rubber farmers who have refused to accept the government's offer.
He said rubber farmers in the district were willing to talk with the government.
Mr Parik also urged the government to go on television to announce details of measures to help the farmers.
The protesters have blocked roads on and off since late last month to demand the government raise its price guarantee for unsmoked rubber sheet to 100 baht per kilogramme and introduce a 6-baht/kg price guarantee for oil palm nuts.
Most rubber farmer representatives earlier accepted the government's offer of a 2,520-baht-per-rai cultivation subsidy.
With the reduced production cost, the government said the farmers will be able to obtain an income of about 90 baht/kg for rubber sheet.