The shots were fired at a bus and a flatbed truck carrying demonstrators to their camp in central Bangkok after they had protested at a government office complex just north of the city.
Erawan emergency services said three men and two women were shot and one of the men had died. Ramathibodi Hospital said 52-year-old Wasan Kamwong died from severe injuries to his head.
The incident underlined the continuing potential for violence in the country's ongoing political crisis. Supporters of Prime Minister Yingluck Shinawatra, fearful that she may be forced out of office by legal manoeuvring, are threatening to take to the streets, opening up the possibility of clashes with anti-government demonstrators.
Since November, Yingluck's opponents have been staging aggressive protests in Bangkok, temporarily blockading and occupying government offices, interfering with registration and voting in a general election in February, and clashing fiercely with police on several occasions.
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Protest-related violence has left 24 dead and hundreds hurt since November, with police among the victims.
The women shot today were on a bus and the men were guards for the protesters who were on a sound truck, said Nasser Yeemah, who heads the guards for the Student and People's Network for the Reform of Thailand protest group. He said he suspected the shots were fired from a tall building at the side of the expressway where the vehicles were travelling.