A videograb shows the police torturing the man.
People who saw the incident, which occurred in the town of Umbauba in the northeastern Brazilian state of Sergipe, published the death of Ginvaldo de Jesus Santos on social media.
His family said he was suffering from schizophrenia.
The shocking clip, which appeared a few days after another case involving police violence, sparked protests in the small town.
Protesters blocked the highway, set tyres on fire and waved signs calling for justice.
At least 23 people were killed earlier in the week during a police raid in Rio de Janeiro’s favela.
According to the Santos family, police approached him while he was riding his motorcycle in the area.
One of his relatives told the G.1 news website that he became nervous when police found schizophrenia medicine in his pocket.
He added, "They threw some kind of gas into the trunk of the car and went to the police station, but he was unconscious. They took him to the hospital, but it was too late.”
The Brazilian Highway Police said in a statement that Santos was arrested because he resisted the police forcefully.