Tossapol Ngamlamai, 35, was shot in ankle by an unidentified gunman as he was standing on the Hua Chang Bridge five minutes before midnight Tuesday, Thai media reported. He also received a head injury.
Amphai Charoenrit, a 55-year old woman from Surat Thani, was shot at her right arm.
The two injured people were admitted to King Chulalongkorn Memorial Hospital.
Former Democrat MP for Uthai Thani Kuldej Puapattanakul, who oversees security at the Pathumwan rally site, said multiple gunshots were heard coming from the Ratchathewi BTS station, opposite Pathumwan intersection, before Mr Tossapol was struck by what appeared to be stray bullets.
The sporadic shooting continued for about 20 minutes, Mr Kuldej said.
A Bangkok Post reporter said gunshots continued to be heard in the area until about 3am.
The video clip below, uploaded to YouTube by a person who went by the name bygon kingdom, shows the shooting at Pathumwan late Tuesday night.