The wealthy family of a protester who was injured and lost sight in one eye during a demonstration has promised all-out legal action.

Tanat Thanakitamnuay, a 29-year-old known as "Hi-so Luk Nat", was hurt during an anti-government protest in Bangkok on Aug 13.

The Thanakitamnuay family on Thursday night announced via Mr Tanat's Facebook account that his injury was apparently caused by the impact of a blunt cylindrical object. Doctors concluded that he had lost sight in his right eye permanently.

The family blames the disability on that day’s crackdown on protesters, saying authorities had failed to follow international practice regarding the firing of tear gas.

The family wrote that Mr Tanat adhered to the principle of peaceful demonstration and intended only to express his opinions, while avoiding any act that could lead to violence or damage. The demonstration on Aug 13, the family wrote, was not violent enough to justify the authorities' violent response.

The Thanakitamnuays vowed they would take all possible criminal, civil and other legal action against individuals and organisations considered as being responsible.

Mr Tanat wrote later on his Facebook account that he would continue to fight for his rights and that protesters should continue to adhere to peaceful action.

He is the scion of a prominent property development family that controls Noble Development Plc, one of Bangkok's biggest builders of upmarket condominiums.

Mr Tanat is also a former member of the Democrat Party and took part in Suthep Thaugsuban's "Bangkok Shutdown" street campaign. He resigned his membership when the party joined the coalition government formed by the Palang Pracharath Party after 2019’s general election.