* SC Asset, once led by the PM, and M-Link are worst-hit stocks
* Shin group, AIS, Thaicom shares also fall for second day
* Social media campaigns cause collateral damage (Adds more stocks, analyst comments, details)
By Khettiya Jittapong
BANGKOK, Feb 20 (Reuters) - Shares in companies linked to the family of Prime Minister Yingluck Shinawatra fell on Thursday after anti-government protesters launched online campaigns to boycott goods and services connected to these firms.
The political movement has weakened the government over the past few months, blocking roads in the capital and forcing ministries to close. Protesters are now using social media like Facebook to target the business interests of Yingluck and her brother, ex-premier Thaksin Shinawatra as well as companies with weak, or even non-existent links, to the family.
SC Asset Corp, the flagship property business of the Shinawatra family and mobile handset distributor M-Link Asia Corp, have been the worst-hit stocks, falling nearly 10 percent in the past two days.
Yingluck was executive chairwoman of SC Asset before running for office in a general election in 2011, which her Puea Thai Party won. The Shinawatra family owns about 60 percent of SC Asset, according to the company's website.
Hundreds of protesters rallied in front of the Shinawatra 3 building in Bangkok, the headquarters of SC Asset, on Thursday.
M-Link is headed by Yaowapa Wongsawat, a sister of Thaksin and the wife of Somchai Wongsawat, who was briefly prime minister in 2008 when another Thaksin-allied party was in power. Anusorn Amornchat, Yingluck's partner, resigned as M-Link's president after she became prime minister.
The Wongsawat family owns 28.6 percent of M-Link, according to the company website.
"This is having a short-term psychological impact. SC has suffered the most because the Shinawatra family has a direct holding in the company," Sombat Aekwanpattana, an analyst at DBS Vickers Securities said.
At the midsession break, SC Asset and M-Link were both down nearly 5 percent. The main index was 1.12 percent lower.
Other stocks affected include telecoms group Shin Corp , founded by Thaksin before he entered politics, its mobile affiliate Advanced Info Service Pcl (AIS) and satellite operator Thaicom Pcl.
Shin Corp shares fell 1.7 percent, while AIS declined 2.8 percent. Thaicom lost 1.9 percent.
The Shinawatra family's sale of its Shin Corp stake to Singapore's Temasek Holdings in 2006, and the way the deal was concluded without taxes, stoked the opposition to Thaksin that culminated in a military coup that toppled his government that year.
He has lived in exile since 2008 to avoid a jail term handed down for abuse of power.
Shin Corp said the company no longer had any connection with the Shinawatra family. "The company has not got involved in politics. The company's founder does not hold any stake in our companies," Wilai Keangpradoo, a spokesperson for the Shin group, said in a statement.
The protesters' list of companies to target includes more than 40 names ranging from universities, hospitals, restaurants and coffee shops to hotels, golf courses and department stores.
TMB Bank, formerly the Thai Military Bank, put out a statement refuting claims in social media that it was owned by the Shinawatra family. It is majority-owned by ING Groep of the Netherlands and the Thai finance ministry. ($1 = 32.56 Baht) (Editing by Alan Raybould and Miral Fahmy)