Pol Col Jatuwat Kingbualuang, the superintendent of Klaeng police station, said the injured were identified as Thanyarat Lamom, Udomsak Ruangpradit, Sitthichai Amnatdej and Alongkorn Tem-iam. They were admitted to Klaeng Hospital and have all been declared safe.
He said the explosion also damaged five cars and two motorcycles parked near the protest site.
Pol Col Jatuwat said the attack occurred when the rally attended by about 500 people almost ended.
Banyat Siankiaw, a security chief in Klaeng, said the blast took place about 15 metres from the protest stage. It was likely that the attack was not meant to cause fatalities but the attackers might want to scare off the protesters, he added.
The rally was organised by the anti-government People's Democratic Reform Committee (PDRC).