Former NBA player MarJon Beauchamp was briefly detained -- then quickly released by police -- after trying to break up a fight between friends ... but, before they let him go, all those around him were in for a treat as he flashed his abs.

See for yourself -- the former Philadelphia 76ers hooper had his body on full display just before 3 AM in Seattle, Washington, while police spoke to him about the incident.

You can see he appeared to be entertaining the police ... even cracking a smile at one point before being let go without incident.

A source familiar with the situation tells us ... MarJon was hanging out with a bunch of high school friends in Seattle's Belltown neighborhood when two of them got into a fight.

We're told ... MarJon tried to defuse the brawl -- which is when one of the guys swung at him, forcing him to defend himself. Another source claims the fight between MarJon's friends was over women.

The cops were called and shirtless MarJon was detained.

A handful of women stuck around the scene, too ... and we have a feeling they didn't look away as they soaked up the sight of the athlete's rock-hard physique.

MarJon is currently in the off-season -- he's a free agent after playing 14 games for the Sixers last year -- but it seems like he's ready to hit the court ASAP!