Brazil's former president Jair Bolsonaro, imprisoned for attempting a coup, was temporarily released on Wednesday to undergo surgery in the capital Brasilia.

Journalists saw a convoy of black cars escorted by motorcycles enter the parking garage of the DF Star hospital ahead of Bolsonaro's surgery on Thursday.

It was unclear how long he would remain hospitalised.

The permission to leave prison granted by Supreme Court Justice Alexandre de Moraes, who oversaw Bolsonaro's trial, comes after an expert review last week stated that the operation was medically necessary.

Bolsonaro, 70, has grappled with health issues since being stabbed in the stomach during the 2018 presidential campaign. He was recently diagnosed with skin cancer.

His surgery on Thursday will repair an inguinal hernia, a protrusion in the groin area due to a tear in the abdominal muscles.

Bolsonaro had undergone another surgery at DF Star in April.

The ex-president began serving a 27-year sentence in late November at federal police headquarters in Brasilia.

In office from 2019 to 2022, he was sentenced by the Supreme Court in September to prison after being found guilty of having led a scheme to prevent President Luiz Inacio Lula da Silva from taking office and to retain power.

He has maintained his innocence, declaring he was a victim of political persecution.

Moraes has ordered a significant security detail to surround the former president during his hospitalisation. (AFP)