"Red Shirt" movement leader Kwanchai Praipana, who played a key role in the 2010 mass rallies in Bangkok, was shot in the shoulder and knee by unidentified persons at his home in northeastern Udon Thani town, a stronghold of Yingluck.
The attack was possibly a politically motivated crime, police were quoted as saying by media reports.
The attack came as anti-government protesters continued to block parts of the capital, ignoring the emergency rule that came into force this morning. The emergency rule gave the government wide-ranging power to control crowds and censor media.
Protesters have vowed to rid Thailand of the political dominance of the Shinawatra clan and alleged that Yingluck is controlled by her fugitive brother, former premier Thaksin Shinawatra who was ousted in a coup in 2006. He lives in self-imposed exile in Dubai.
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The ongoing unrest, sparked by an amnesty bill that could have facilitated Thaksin's return to the country, has pitted Bangkok's middle class and royalist establishment against the mostly poorer, rural supporters of Yingluck and her brother.
Yingluck, who signalled her determination to go ahead with the February 2 snap polls, yesterday said her government has no intention of confronting the protesters.