Military police in the northeastern state of Ceara say the attempted heists began around 2.30am yesterday in the central area of the city of Milagres.
The robbers at one bank had taken several people hostage when police surrounded the area. A firefight broke out, leaving several dead.
Authorities have yet to identify the dead. The mayor of Milagres said two children, between ten and 13 years old, were killed.
It was not immediately clear how the hostages were taken.