An OPP officer is standing trial in Thunder Bay on charges of assault after an investigation into injuries suffered by a 42-year-old First Nations man who was in police custody.

The complainant reported he had been punched by Constable Brian Bellefeuille while he was in his cell at the Greenstone OPP detachment in February of last year.

On Monday the prosecuter laid out the case, saying that Gary Megan was heading for a bar in a taxi when he gave the finger to a police officer.

Megan was arrested by the OPP several hours later for public intoxication.

The crown said he was handcuffed in a cell with his hands behind his back when Bellefeuille pushed him to the floor.

Bellefeuille has also been charged with public mischief for interfering in a police investigation.

The court heard from the doctor who treated Megan for broken bones in his eye-socket, as well as the OPP officer who was on the receiving end of Megan's rude gesture.

The trial is expected to wrap up Tuesday.