Police Lt. Thanabhum Newanit said assailants in a pick-up truck attacked a Saturday night rally held by the People's Democratic Reform Committee in the province of Trat, about 300 kilometres (180 miles) east of Bangkok.
He said the attackers shot into the crowd and two explosive devices went off. It was not clear if the protest group, which uses armed guards, fought back.
Protest-linked violence has increased recently, in the forms of fighting with police and attacks by unknown parties on the protest sites, which are mainly in the capital, Bangkok.