The singer-songwriter was onstage introducing fellow musician AP Dhillon at the annual event in Edmonton, Canada Monday night ... when things suddenly took a bizarre turn.

Check out the video ... the woman sauntered onto the stage with no security in sight, prancing around with a message scrawled across her back.

The message read, "SAVE THE GREEN BELT," a reference to a controversial residential development plan spearheaded by the Ontario government.

Avril played it cool at first and ignored the woman, but then decided enough was enough. She turned to the woman and barked, "Get the f**k off. Get the f**k off, b***h."

Security finally showed up and escorted the woman away. It's not clear if she was arrested.

Later on, Avril joked about the strange encounter during her acceptance speech for the TikTok Juno Fan Choice Award. She said, "Now nobody try anything this time or the Canadian's gonna come out of me and I'll f**k a b***h up."

The audience ate it up. One fan told a reporter Avril did "a great job handling" the disruptor. Another person said he got a good chuckle out of it, while acknowledging Avril had initially kept her cool. A third person commended Avril for doing a fantastic job in "fighting through" the disturbance.

It marked the latest drama in Avril's life. As we reported, she broke off her engagement with Mod Sun and is now dating Tyga. Avril and Tyga were recently seen gallivanting around Paris during Fashion Week, locking lips and holding hands. We're told their growing relationship is no publicity stunt, but the two haven't put a boyfriend/girlfriend label on it yet.