The shooting happened in Guamá, a poor and violent neighbourhood of the city in the Amazon region.
Local media speculated that the shooting was carried out by a gang in retaliation for the deployment in March of federal forces to Belém.
So far, no arrests have been made in connection with the crime.
Military police said seven armed men had arrived at Vanda's bar in a car and on three motorbikes on Sunday afternoon, local time.
Witnesses said the masked assailants appeared to shoot at random, killing five women and six men. One person was injured and is under police protection.
While the owner of the bar was reportedly among those killed, many people were able to flee through a back door.
Guamá is one of the most violent neighbourhoods in Belém, the capital of northern Pará state.
It was one of seven neighbourhoods to which federal forces were deployed in March with the aim of helping the local police force come to grips with spiralling crime.
The governor of Pará state, Helder Barbalho, said that the shooting would not make him change his approach to security.
"We will not back down. We will remain firm and work to guarantee the right of the population to have peace and security," he said.
Belém was the third most violent city in Brazil in 2018 after Natal and Fortaleza, according to figures published by the Mexican NGO Security, Justice and Peace.
Rising crime levels were one of the key issues in last year's election which brought far-right politician Jair Bolsonaro to power.
Mr Bolsonaro campaigned on a promise to empower "good citizens" to defend themselves from criminals, allowing them to own and carry guns.
Earlier this month, he signed an executive order which - once it comes into effect next month - will make it easier to import guns and which will increase the amount of ammunition Brazilians can buy in a year.