* Government supporters plan rally for Sunday
* Protest leader says "let them come"
* Indonesian trade official sees signs of Thais dumping rice
By Amy Sawitta Lefevre
BANGKOK, Feb 20 (Reuters) - Protesters campaigning to oust Thai Prime Minister Yingluck Shinawatra turned to Facebook and other social media to attack businesses linked to her family on Thursday and rallied outside the offices of a property developer whose share price tumbled.
Yingluck's government appears increasingly hemmed in by opponents and the judicial system, lacking the fiscal powers to fund key policies and warned by a court on Wednesday that it cannot use a state of emergency to disperse protesters.
Four protesters and a police officer were killed on Tuesday, in the deadliest clashes since the unrest began in November, when police attempted to reclaim sites near government buildings that have been occupied for weeks.
The protesters are seeking to unseat Yingluck and stamp out what they see as the malign influence of her brother, Thaksin Shinawatra, a self-exiled former premier regarded by many as the real power behind the government.
About 500 protesters gathered outside the Bangkok offices of SC Asset Corp, a property developer controlled by the Shinawatra family, waving Thai flags and blowing whistles.
"We will hamper all Shinawatra businesses," protest leader Suthep Thaugsuban told supporters at a rally on Wednesday night. "If you love your country, stop using Shinawatra products and do everything you can so that their business fails."
Yingluck was executive chairwoman of the company before being swept to power in a landslide election victory in 2011.
Shares in SC Asset fell 4.3 percent on Thursday, following a similar fall the previous day. Shares in M-Link Asia Corp , a mobile handset distributor with links to the Shinawatras, have also lost almost 10 percent over the past two days.
Anusorn Iamsa-ard, deputy spokesman for Yingluck's Puea Thai Party, said the building the protesters targeted had nothing to do with her.
"The prime minister has no shares in SC Asset," he said.
"The thing that will worry her is the impact of the protest today, and the fact employees of the office tower had to leave, on the Thai economy overall. But the campaign against Shinawatra businesses will not weaken her resolve to see Thailand through this crisis."
Other stocks affected include telecoms group Shin Corp , founded by Thaksin before he entered politics, and its mobile affiliate Advanced Info Service Pcl (AIS) . Shin Corp said it no longer has any connection with the Shinawatra family.
"We should change our service provider and stop using AIS ... it's easy to change SIM cards these days," said a post on the "No Thaksin" Facebook page.
"I ask the people to stop using the tyrant Thaksin's products. Stop using AIS ... so we can teach Thaksin a lesson ... and so he can know that hell really exists," a user posted in response.
Yingluck has headed a caretaker government with limited spending powers since calling a snap election in December. Voting on Feb. 2 was disrupted by her opponents, and it could be months before a new government can be installed.
An anti-corruption agency this week filed charges against her over a soured rice subsidy scheme that has stoked middle-class anger and left hundreds of thousands of farmers, her natural backers, unpaid.
More than 1,000 farmers, many riding in farm trucks, were travelling in convoy towards Bangkok from the rice-growing central plains and were due to reach the city overnight or on Friday.
Chada Thaiseth, a former member of parliament, said he would lead farmers on Friday to Bangkok's Suvarnabhumi Airport, blockaded for eight days by royalist, anti-Thaksin "yellow shirts" in 2008.
"We are not sure where we will set up camp, but we will not leave the capital until we are paid for every grain of rice sold," Chada told Reuters.
Yingluck's "red shirt" supporters plan a rally in Nakhon Ratchasima, northeast of the capital, on Sunday, when they will decide what to do next.
"We are not saying that we want to come out and fight, but it seems that Suthep is challenging us red shirts to come out and face off," spokesman Thanawut Wichaidit said.
"Let them come," Suthep said in a midday speech to protesters in Bangkok. "Do they dare?"
The protests are the biggest since deadly political unrest in 2010, when Thaksin's red shirt supporters paralysed Bangkok in an attempt to remove a government led by the Democrat Party, now the opposition.
More than 90 people were killed and 2,000 wounded when Suthep, at the time a deputy prime minister, sent in troops.
The protests are the latest instalment of an eight-year political battle broadly pitting the Bangkok middle class and royalist establishment against the mostly rural supporters of Yingluck and Thaksin.
Demonstrators accuse Thaksin of nepotism and corruption and say that, prior to being toppled by the army in a 2006 coup, he used taxpayers' money for populist subsidies and easy loans that bought him the loyalty of millions in the country's populous north and northeast.
Yingluck has continued her brother's policies, but the rice scheme, which paid farmers way above the market rate, has proved ruinously expensive and has run into funding problems.
Thai rice prices fell 15 percent this week as the government rushed to sell some of its record stockpiles to prop up the scheme. An Indonesian trade official said there were "indications" Thai suppliers were dumping the grain in Indonesia.
Thailand's anti-corruption body began an investigation last month into the rice scheme and said on Tuesday it was filing charges against Yingluck. She was summoned to hear the charges on Feb. 27.
Police have made no further moves against the protesters, whose barricades and encampments are still blocking several big intersections in central Bangkok, since gun battles erupted during a security operation on Tuesday.