Some 20 detainees were injured when about 300 delinquents in a juvenile detention centre in Samut Prakan province clashed on Friday evening, police said.
The fight inside the Ban Karuna detention centre in Bang Plee district happened at 5pm.
Officials took over half an hour to control the situation, Fourteen detainees had to be rushed to nearby hospitals. Two of them were severely injured after being stabbed with pointed iron bars.
Samut Prakan police chef Pol Maj-General Thammanoon Traitippayapong said the clash occurred inside the locker room at the bathing zone.
He said 367 juvenile delinquents are being detained there. He said two groups, which have a long-running feud, clashed with weapons they had made and hidden.
He said police would later help officials of the centre conduct a thorough search for hidden weapons.
An injured boy said his group of detainees in a smaller building were often harassed by another group staying in a bigger building.
The boy said the other group attacked his group during bathing time.
Samut Prakan Governor Chartchai Uthaiphan, who inspected the centre after the clash, said he had instructed security officials at the centre to prevent more quarrels among the youths.