The armed group, whose members were wearing masks to cover their faces, arrived in cars and a motorbike before shooting dead six women and five men in the bar in a poor, violent neighbourhood in the capital of Para state, said G1 news website.
There was no information regarding possible motivation for the crime and no arrests had been made by early Sunday evening. The identities of the victims were not given.
Some videos taken right after the shooting were posted on social media, showing slain people laying on the ground and a dead woman on top of the bar's counter.
The killings took place in the Guamá neighbourhood, one of the seven most violent spots in the metropolitan Belem area where federal troops were sent in March to boost security.
There were 756 violent deaths in Para state in the first quarter.