Gang members from one prison block invaded another part of Altamira jail in Pará state, the BBC reported.
Officials giving a news conference said 16 of the dead were decapitated. Other reports said part of the prison was set on fire, suffocating many.
Two prison officers who were taken hostage have since been freed.
The violence began at around 7 a.m. (local time), and ended at around noon, officials said.
Video reported to be from the prison carried by Brazilian media showed smoke billowing from at least one prison building, while another clip appeared to show inmates walking around on the rooftops.
The two rival gangs involved have not been named by authorities.
The prison in Altamira where Monday's violence broke out has a capacity of 200, but was occupied by 311 prisoners, Brazilian news outlet G1 said.
Violence in Brazilian prisons is not uncommon. The country has the world's third-largest prison population of some 7,00,000 people, and overcrowding is a widespread problem.
Clashes between rival gangs are frequent, as are riots.
In May, 40 people were killed on the same day in four different prisons in Manaus in Amazonas state - a day after 15 died in prison fights in the area.
Brazil's far-right President Jair Bolsonaro has vowed to bring in tighter controls in prisons, while building more across the country.
But this will not be straightforward as most prisons are controlled at the state level.
( With inputs from IANS )