The wild scene happened just before halftime of L.A.'s loss to the Niners in Northern California ... when a protester hopped over Levi's Stadium railing and began running wild on the turf.

The man -- holding a smoke bomb that filled the air with pink fumes -- managed to evade stadium staffers for several moments ... until he made his way over to the Rams' sideline.

That's where Wagner buried his shoulder straight into the dude's chest -- leveling him to the grass. Security guards were then able to pile on the guy ... and he was removed without further incident.

The Manning brothers -- Peyton and Eli -- found the whole situation pretty funny while they were in the middle of broadcasting the action ... but Wagner said afterward he had legit concerns that the fan had nefarious intentions.

"We don't know what they're carrying in their pockets," the 32-year-old told media members.

"That's just keeping it safe," he added. "You don't know what that fan got or what they're doing."

Animal rights group Direct Action Everywhere took credit for the scene after the game ... explaining the man, and another protester who had been taken down before reaching the pitch, had hit the field in order to raise awareness for a trial involving the alleged theft of pigs from a factory farm.

The org. said the protesters were cited by police and ultimately released from custody. It added that the man who was tackled by Wagner suffered a burn on his arm.