The 33-year-old martial arts legend was visiting the nightclub with friends when another guest approached the group's table.
According to reports, the stranger threatened Leandro Lo and his friends, among other things, with a bottle.
Lo wrestled the man to the floor to calm the situation, according to information to AFP.
As Lo's friends helped them both up, the stranger suddenly pulled out a gun and shot Leandro Lo in the head.
Although he was quickly taken to hospital, he is not judged to be able to survive and he has been declared brain dead.
According to the authorities, the shooter has been identified, according to several reports it is a policeman who was not on duty.
Leandro Lo won his eight world championship titles in five different weight classes between the years 2012 and 2022.