Updated

Rubber farmers clashed with security forces in southern Thailand as protesters torched cars and police fired tear gas in the latest unrest to shake the politically turbulent kingdom, officials said on Friday.

More than two dozen police officers were injured and 11 protest leaders were arrested as the demonstration in Prachuap Khiri Khan province over falling rubber prices turned violent late on Thursday, according to the authorities.

Two vehicles, apparently belonging to local media, were set ablaze during the clashes, which flared after some 400 farmers tried to block a major highway to the south of the country, provincial police commander Major General Thanet Soonthornsuk told AFP.

"Twenty-five police officers were wounded. Two of them are in a serious condition after they were hit on the head by rocks," he said, adding that demonstrators had thrown acid at him.

Thailand is the world's top exporter of natural rubber and mounting anger among the kingdom's rubber farmers over their falling incomes poses a fresh challenge to Prime Minister Yingluck Shinawatra's two-year-old government.

In recent days angry rubber farmers have blocked roads, train lines and even -- briefly -- the entrance to an airport in Thailand's south where plunging global prices for the commodity have hit the local economy hard.

The region is a heartland of the opposition Democrat Party.

Yingluck's government has so far rejected demands to guarantee a rubber price of 120 baht ($3.7) per kilo -- which farmers later reduced to about 90-100 baht -- about 50 percent higher than the current price on world markets.

Instead it has stuck to an offer to pay farmers 1,260 baht per rai (0.4 acres, 0.16 hectares) of rubber plantation to help with production costs, along with funds to boost the efficiency of rubber processing -- an offer rejected by the protesters.

Deputy Prime Minister Pracha Promnog said he would travel to the south later Friday to meet protest leaders.

"We will open talks with them at all levels," he told reporters, without revealing if the government was ready to make concessions.

Thailand has been rocked by several episodes of civil unrest in recent years, with both supporters and opponents of Yingluck's brother -- fugitive former premier Thaksin Shinawatra -- taking to the streets.

In 2010 two-month demonstrations in Bangkok by the pro-Thaksin "Red Shirts" drew 100,000 protesters at their peak before being crushed in a military crackdown under a previous government.

More than 90 people, mostly civilians, were killed during the demonstrations and nearly 1,900 were injured in Thailand's worst political bloodshed in decades.