"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 Wednesday morning. The emergency rule gave the government wide-ranging power to control crowds and censor media.
The protesters want an unelected "people's council" to govern the country until political reforms are carried out.
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.
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, on Tuesday said her government has no intention of confronting the protesters.
The areas covered by the 60-day emergency decree have already been placed under tougher-than-normal security under the Internal Security Act.
A defiant protest leader Suthep Thaugsuban, who faces an insurrection charge in connection with the unrest, questioned whether the decree was justified and vowed to keep up the demonstrations.
The opposition Democrat Party, which supports the protests, has said it would boycott the February 2 polls in a bid to end the unrest. The Election Commission wants the polls to be postponed.
Nine people have been killed and hundreds injured in violence since the protests began in early November. The political deadlock is the worst since 2010 when thousands of pro-Thaksin "Red Shirt" protesters occupied key parts of Bangkok. Over 90 people, mostly civilians, died over the course of a two-month sit-in.
Meanwhile, a spokesperson of the anti-government movement People's Democratic Reform Committee said their protest would continue despite the emergency rule.
PDRC leader Akanat Promphan said he believes the state of emergency would prompt more people to join the protest.
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