Kwanchai Praipana, a local radio presenter who played a large role in Bangkok protests in 2010, was wounded at his home in Udon Thani in the north.
It came as anti-government protesters continued to block parts of Bangkok to force the prime minister to resign.
The emergency decree gives the government wide-ranging powers.
Imposed late on Tuesday, it covers Bangkok and three surrounding provinces. It gives the government the power to control crowds and censor media, but it remains unclear how it will be enforced.
The protesters – who began their campaign in November – say PM Yingluck Shinawatra’s government is being influenced by her brother, exiled former leader Thaksin Shinawatra.
The protesters want an unelected “people’s council” to run Thailand until its political system is changed.
Yingluck Shinawatra has refused to step down and has called an election on February 2, which the opposition are boycotting.
The emergency declaration follows incidents of violence during protests, with both the pro-government and anti-government sides blaming each other for attacks.
At least nine people have died since the wave of protests started last year.
Kwanchai Praipana, a prominent leader of the “red shirts” who support Thaksin Shinawatra and the current government, was wounded in the leg and shoulder while standing outside his home on Wednesday.
Police Colonel Kowit Tharoenwattanasuk told Reuters news agency that unidentified people fired shots from a pick-up truck.
He added that the attack was possibly a “politically motivated crime”.
The “red-shirt” government supporters – who shut down Bangkok in 2010 – have for the most part stayed away from these protests. But observers fear that violence could erupt if a trigger brought them out onto the streets.
In Bangkok, meanwhile, there was little change seen on the streets in the first few hours of the state of emergency, with anti-government protesters continuing their blockades in the city centre.
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