Mr Samran said in an interview with Democrat Party-owned BlueSky satellite television that this happened about 3am. The explosion shattered seven glass windows and damaged his car's radiator. Nobody was hurt.
Mr Samran was at the Ratchadamnoen rally site at the time of the attack. He was informed of it by his wife.
Police examined the scene this morning. It was not yet known how many people were involved in the attack.