Fiji Opposition MP Pio Tikoduadua is still being held at a police station in Suva.
The National Federation Party president was arrested at his home early Tuesday over a video post on social media..
NFP leader Biman Prasad said despite calls for Mr Tikoduadua's release, police were still questioning him.
"Mr Tikoduadua has a set of medical conditions such as high blood pressure including heart issues. The lawyers have articulated those to the police commissioner calling on him to release Mr Tikoduadua."
Mr Prasad said police could've arranged for Mr Tikoduadua to be interviewed without being locked up in a cell.
Police spokesperson Ana Naisoro confirmed Mr Tikoduadua was being questioned at CID headquarters in Suva.
But Mr Prasad condemned the arrest and said the heavy-handed police tactics were uncalled for.
He claimed police were upset about a video on Mr Tikoduadua's Facebook page.
He said in the video, Mr Tikoduadua had set out allegations of police brutality in the village of Naqia in Tailevu.
"As a result of this, a young man is alleged to have been thrown off a bridge and is in hospital with serious injuries."
Mr Prasad said the incident had received wide publicity and police said they were investigating.
He said Mr Tikoduadua's lawyers had written to the police commissioner calling for his release.
"No he hasn't been released. He was locked up again in the Totogo Police Station cell last night. One of our members visited him and delivered his breakfast. But he's in good spirits."
Local law stated a person could be held in custody for up to 48 hours without charge.