Local newspaper Tribun Bali published a report late Thursday afternoon saying that the clash was allegedly still ongoing.
A prison employee, who declined to be named, said the clash started at about 4pm.
The brawling is believed to be occurring between different "organisations", or gangs, in the prison population.
The source inside the prison also said the clash has even already resulted in a fatality.
"There is one death. Prison officers are still inside," the prison employee told Tribun.
Police have stepped up security in the area and there have been some road closures surrounding the prison, reports say.
SBS News has been unable so far to verify the claims.
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