The shooting heard after the protest at SCB Park ended on Wednesday night involved students from rival vocational schools who worked as guards during the rally, according to police.
Pol Maj Gen Piya Tawichai, deputy commissioner of the Metropolitan Police Bureau, said on Thursday the incident occurred after the rally ended outside the headquarters of Siam Commercial Bank (SCB).
Gunshots were heard and an explosion, possibly a firecracker, about 10pm.
The deputy commissioner said a young man with one group of vocational students fired shots at a group of rival students. One person was wounded, taken to Praram 9 Hospital and admitted to intensive care.
Witnesses said the students were from Minburipolytechnic Technical College in Bangkok and Pathumthani Technical College in Pathum Thani province.
The students had argued while working as rally guards on the SCB Park side of Ratchaphisek Road. Their dispute continued after the rally ended and they had crossed to the opposite side of the road, and the shooting occurred there.
"There are surveillance camera recordings and videos taken by reporters and other people who were in the area," Pol Maj Gen Piya said.
"I can confirm that both groups were guards for the Ratsadorn group and argued over their jobs at SCB. No outsiders were among them." The Ratsadorn group leads the anti-government demonstrations.
He warned organisers to ensure their rally guards were unarmed, otherwise any future quarrel could result in injury to innocent demonstrators.
Pol Maj Gen Piya also said police had already removed all barricades installed on roads near Government House and the Crown Property Bureau on Wednesday.
Police later reported that two people had actually been injured during the shooting on Wednesday night.
The second was the gunman, who was given a sound kicking and later taken to Saint Louis Hospital.
On Thursday afternoon, Pol Maj Gen Jirapat Phumjit, deputy commissioner of the Metropolitan Police Bureau, identified the man who was shot as Prapakorn Saksritao, and the gunman as Pasapong Kul-amornkan, 25. Both were former students of the two vocational schools.
He said one group of rally guards first threw a ping pong bomb and fled. The other group chased them, and Wanchai fired a revolver. The clash resulted from a personal conflict and doctors had not allowed police to question the two men, he said.