The Brazilian surfing veteran had been hitting the massive waves at Praia do Norte beach in Nazare on Thursday, practicing his tow-in ... when authorities say he fell off his board.

According to the Portuguese National Maritime Authority, rescuers found the 47-year-old in cardio-respiratory arrest ... and immediately started "resuscitation maneuvers on the sand."

However, Freire didn't make it despite the life-saving attempts.

The surfing world was rocked by the news ... with Nic von Rupp mourning the loss in an emotional social media post.

"We surfed all day, it was perfect, clean, barrels, Márcio was stoked had that huge smile. With that smile is how I’m going to remember him."

Added Hawaiian surfing star Matt Meola in an Instagram post of his own, "Man what a sad day for surfing. Hard to imagine somebody who seemed so big and invincible is gone."

Freire -- one of the three "Mad Dogs" surfers who victoriously rode the "Jaws" wave in Hawaii -- had been seen just moments before his death smiling and enjoying the waves.