About 500 farmers from Nakhon Si Thammarat and neighbouring provinces staged a roadblock in Cha-uat district about 2pm to call on the government to prop up plummeting crop prices.
It was their third demonstration in a month after their previous two rallies, on Aug 3 and 19, failed to yield any concrete government response.
The farmers want the rubber milk price to be shored up to 120 baht a kilogramme and oil palm fruit to 7 baht/kg. The rubber milk price was at 68 baht/kg yesterday while oil palm fruit was at 3.60-4.10 baht.
They also demanded authorities not take legal action against protesters.
About 1,300 police officers and defence volunteers were deployed at the rally site yesterday.
Nakhon Si Thammarat governor Viroj Jiwarangsan arrived at the scene and instructed the protesters through loudspeakers to reopen the road, but the they refused to end the blockade as their numbers grew to more than 600.
About 3.30pm, police decided to disperse the protesters, who resisted by throwing rocks and firing slingshots at the officers.
Forty-three people, including five police officers, were injured in the clash, which lasted about 15 minutes. The injured were rushed to Cha-uat Hospital.
Following the violence, the number of farmers increased to about 1,000 and the blockade continued.
About 6pm, riot police tried again to break up the protest.
However, the police were forced to retreat after the farmers threw water bottles and other missiles at them.
No injuries were reported in the second clash.
Nakhon Si Thammarat police chief Pol Maj Gen Ranapong Saikaew later ordered police to back away from the protest site, allowing the farmers to continue their blockade.
The clashes prompted Democrat MP for Nakhon Si Thammarat Witthaya Kaewparadai to raise the matter during the budget debate in parliament.
The MP called on Prime Minister Yingluck Shinawatra to order the Nakhon Si Thammarat police chief to withdraw forces from the scene to reduce tensions and to prevent the situation from escalating into bloodshed.
Mr Witthaya also called on Ms Yingluck to immediately transfer the provincial police chief and Mr Viroj out of the province as he said the two had failed to control the situation.
Democrat Party list-MP Jurin Laksanavisit said the rubber and oil palm planters have repeatedly asked the prime minister to look into their plight but their calls have fallen on deaf ears. "Parliament should be discussing farmers' problems instead of conducting lengthy debates on the amnesty, the charter amendment and budget bills," he said.
Mr Jurin proposed adding the slump in farm product prices as a topic to be addressed in Thursday's House session agenda and called on Ms Yingluck to attend the sitting.
Deputy Agriculture and Cooperatives Minister Varathep Ratanakorn responded that the situation was under control and the prime minister had already instructed national police chief Adul Saengsingkaew and Interior Minister Charupong Ruangsuwan to closely monitor the situation.