Around 500 protesters armed with slingshots clashed with riot police firing tear gas on Monday after blocking a highway in southern Thailand in a bid to force the government to increase a subsidy for rubber farmers.
At least 30 people were injured.
Most farmers groups have agreed in principle to the government's offer of a 21.2 billion baht ($665 million) subsidy, which works out at around 10 baht per kg.
On top of the current market price of around 80 baht, that would give farmers 90 baht ($2.82) for each kg of rubber they produce, but the farmers protesting on Monday said they wanted income of 120 baht per kg.
"Police officers were injured in the clash and two guns were seized by protesters while 10 cars were smashed and burnt," said Wirot Chiwarangsan, governor of Nakhon Si Thammarat province, where the clash took place.
The injured included 27 police officers, according to the National Institute for Emergency Medicine in Nakhon Si Thammarat.
"Protesters used slingshots loaded with marbles and stones to fire at police, injuring many officers. Police saw the situation escalating, so they decided to use teargas," Somchai Auamthanom, a deputy police commissioner, told Reuters.
Tens of thousands of rubber farmers, angered by steep falls in prices, took to the streets earlier this month, blocking roads and railways, in one of the biggest challenges faced by the populist government of Prime Minister Yingluck Shinawatra.
Finance Minister Kittirat Na Ranong told reporters the government would not renegotiate the rubber subsidy.
The group opposed to the deal offered by the government last week had closed off a major road bridging southern and central Thailand for a period on Saturday.
The group says it will take its protest to the area around parliament in Bangkok if its demands are not met within a week.