The four labourers - Phon Mueangjan, Meemak Saranyoo, Bopribun Suksan and Kaeokone Baeng - were charged with offences against public safety, theft and obstruction of official duty. Public prosecutors identified them through video footage.

The court ruled that the poor-quality video footage showing blurred images could not be considered convincing evidence that they had incited the riot and set fire to the management office in their living quarters, the agency reported.

As a result, the court found them not guilty.

In a separate ruling, the court passed a six-month sentence on Yen Shih-hua, general manager of Huapan Company, and four of his staff members for providing video gambling machines in the labourers' dormitories.

Huapan was responsible for the management of some 1,700 Thai labourers working on the Kaohsiung rapid-transit railway project.

In its ruling, the court asked prosecutors to find out whether Yen and other Huapan employees had enslaved and exploited the Thai workers, according to the news agency.

The Kaohsiung riot occurred when some of the Thai workers protested at poor living conditions and restrictions placed on them by Huapan.

Mounting pressure and criticism have prompted the Kaohsiung city government and Kaohsiung Rapid Transit Corporation to improve the living conditions of the Thai workers, who now live in comfortable dormitory buildings in the Hsiaokang district of Kaohsiung City, the Central News Agency said.