Ex-NFL Star Seth Joyner -- EXPLODES ON YOUTH FOOTBALL TEAM ... Tough Love or Too Far?

A former Super Bowl champ is UNDER FIRE from parents in a youth football league -- who say the raging pep talks he gives the team are abusive ... but the ex-NFL star says it's just tough love.

The man at the center of the controversy is Seth Joyner -- a 3-time All Pro linebacker -- who now coaches an American Youth Football team in Tempe, AZ comprised of 12-year-olds.

We're told league officials were recently contacted after Joyner went ballistic on the kids during a recent game -- even dropping an F-bomb in an effort to get his team to play better.

TMZ Sports obtained footage of one of the rants ... Joyner is clearly fired up ... and at one point, threatened to "forfeit the f***king game." It's intense.

We spoke with a former coach who worked with Joyner -- who tells us the rant wasn't an isolated incident -- and his players are scared. "The kids are terrified of him. He routinely berates them. The guy is terrible."

But Joyner -- along with several parents -- sees it differently ... telling us he's "passionate" because he wants to get the best out of his players ... and believes he's getting results.

Though Joyner did admit he can sometimes cross the line -- "Do I lose control from time to time?

Joyner says he's spoken with several parents about his coaching methods -- and says "most are completely OK with it."