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.

Around 500 protesters gathered outside the north Bangkok offices of SC Asset Corporation, 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 Wednesday night rally. “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 5 per cent on Thursday, following a similar fall the previous day. Shares in M-Link Asia Corporation, a mobile handset distributor with links to the Shinawatras, have also lost almost 10 per cent over the past two days.

Problems continue to mount for Yingluck, who has headed a caretaker government with limited spending powers since calling a snap election in December. Voting on February 2 was disrupted, 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 central plains, the main rice-growing area, on Thursday.

Chada Thaiseth, a former member of parliament, said he would lead farmers to Bangkok’s Suvarnabhumi Airport.

“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 said.

It was unclear whether the farmers intended to protest at the airport, which was blockaded for eight days by royalist, anti-Thaksin “yellow shirts” in 2008.

Watch: February 18 video shows grenade hurled at police phalanx (Video contains violent images and strong language)

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 per cent this week as the government rushed to sell some of its record stockpiles to prop up the scheme. Thailand’s anti-corruption body began an investigation last month into the rice scheme and said on Tuesday that it was filing charges against Yingluck. She was summoned to hear the charges on February 27.

Yingluck said in a post on her official Facebook page that the National Anti-Corruption Commission (NACC) should not rush an investigation, noting that similar complaints against the previous administration were still under investigation.

“In my position as prime minister and head of the National Rice Policy Committee, I am responsible for policy implementation but, as with all policies, the execution is the responsibility of government officials,” she said.

“I reaffirm that I am innocent of the accusations by the NACC,” Yingluck said on her official Facebook page.

“Even though I am accused of criminal charges and face removal [from office], which were the wishes of people who want to overthrow the government, I am willing to co-operate to establish the facts,” she added.

The NACC says Yingluck ignored warnings that the rice scheme was fostering corruption and causing financial losses.

Her critics say the controversial scheme, which guarantees farmers above-market rates for rice, has encouraged corruption, drained the public coffers and left the country with a mountain of unsold stock.

They accuse her billionaire family of using taxpayers’ money to buy the loyalty of rural voters through such populist policies. Yingluck said she was simply trying to improve the lives of farmers.

The opposition demonstrators want Yingluck to step down and make way for a temporary unelected government that would oversee loosely defined reforms to tackle corruption and alleged vote-buying.

In another legal setback to Yingluck, a Thai Civil Court on Wednesday ordered the government not to use force against peaceful protests, limiting the authorities’ scope to deal with opposition rallies that have descended into violence on several occasions.

New York-based Human Rights Watch accused both sides of using live ammunition in clashes on Tuesday in Bangkok’s historic district in which five people were killed and dozens wounded.

The government has said security forces used only rubber bullets and not live ammunition.

Yingluck’s “red shirt” supporters plan a rally in Korat, 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 at a rally site near Bangkok’s central oasis of Lumpini Park. “Do they dare?”

The protests are the biggest since deadly political unrest in 2010, when the red shirts 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.

The court dismissed a case brought by protest leaders who wanted a 60-day state of emergency announced last month to be declared illegal, but added the government was “not allowed to use clauses in the state of emergency to disperse".

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.