'Impractical Jokers' Joe Gatto Defends Woman From Anti-Fur Activist On Video

Joe Gatto was a knight in shining armor to a woman who was being berated over a fur coat ... he's on camera coming to her defense and calling her foe a "piece of s***."

The heated exchange played out north of the border in Montreal ... and the video picks up with a woman in a fur coat being shamed over her fashion choice as she waits to cross a busy intersection with her two dogs.

Joe, the former "Impractical Jokers" star who is currently on a comedy tour ... walks over and asks the woman if she needs help, telling the anti-fur activist to quit harassing her and "stop being a piece of s***."

Joe was Johnny-on-the-spot here ... the woman's clearly flustered and is trying to explain why the fur coat doesn't make her a terrible person, and the guy's being pretty combative.

The fur debate doesn't seem to be the issue for Joe -- it's snowing out and he's not wearing a fur coat to stay warm himself -- rather, he didn't like the way the man was going after the woman.

Frankly, in this day and age, ya never know what these folks are gonna do.

The anti-fur activist Len Goldberg tells TMZ... "I'm saddened that Joe Gatto, who represents himself publicly as an animal lover, would call me a "piece of s***" when all I did was repeatedly ask this woman why she supports skinning beaver but not dogs for a status statement, instead of letting her change the subject as all fur-wearers do when I pose this question. I'm sad Joe doesn't regard these beaver, who were crushed to death for a fur coat, as being the real victims."

Joe is a big advocate for adopting dogs, at one time boasting 12 of his own ... so it's not like he's necessarily pro-fur -- he just didn't like the way the lady was treated.