Videos of a 38-year-old Brazilian man being asphyxiated with gas in the boot of a police car after getting stopped by highway patrol has caused outrage across Brazil.

The images captured on phone camera showed police officers bundling the handcuffed man into the boot of their SUV, releasing a gas cannister inside and leaning down on the boot door until his screams abated.

Brazil's federal police said in a statement yesterday they had opened an inquiry to investigate the circumstances of the death.

The death of Genivaldo de Jesus Santos, described by his family as suffering from schizophrenia, were shared on social media by onlookers in the town of Umbaúba, in the northeastern state of Sergipe.

The images, coming just days after another case involving police violence, sparked horror across social media and protests in the small town, with demonstrators closing the road, setting tyres on fire and waving signs calling for justice.

According to the victim's family, Santos was approached by the officers while riding his motorcycle through the area. He became nervous when the officers found his schizophrenia medicine in his pocket, Wallyson de Jesus, a nephew of Santos', told news website G1.

"They threw some kind of gas inside the trunk and went to the police station, but my uncle was unconscious. They took him to the hospital, but it was already too late," he said.

In a statement, Brazil's Federal Highway Police (PRF) said Santos was arrested because he actively resisted an approach by police.

It said that due to his "aggressiveness ... immobilisation techniques and instruments of less offensive potential" were used to restrain him.

Santos became ill during the trip to the police station and was then taken to a nearby hospital, where he was pronounced dead, according to the PRF statement.

The statement did not mention the two officers trapping him inside the back of their car as white gas was seen spewing out, but said the PRF had opened an inquiry to probe the officers' conduct.

An autopsy by the state forensic medical institute, seen by Reuters, found that Santos died from mechanical asphyxia.

"This obstruction can occur through several factors, and at this first moment it was not possible to establish the immediate cause of the asphyxia, nor how it occurred," said the report.

Meanwhile, bodies from a Rio de Janeiro police raid that left more than 20 alleged drug traffickers dead show signs of torture and summary execution, a senior lawyer has said.

The incident is the another case of suspected police brutality to shock the South American nation.