WWE star R-Truth is taking serious heat for lighting up a cigarette on "RAW" last night ... in front of a row of kids -- and now, the biggest anti-smoking activist on the planet is DEMANDING he apologize.

The drama began when Ron "R-Truth" Killings hit up the audience for a smoke ... and actually LIT UP right in front of them ... including several children.

But the on-screen cig-fest didn't sit so well with anti-smoking activist Patrick Reynolds (pictured right) -- Executive Director of TobaccoFree.org -- who has officially issued a challenge to the wrestler ... to publicly apologize for the smoking stunt.

Reynolds -- ironically the grandson of tobacco magnate R.J. Reynolds -- tells TMZ, "For him to smoke in front of [children] is irresponsible. At best, it's thoughtless, and at worst, if he calculated appealing to kids by posing as a bad-boy outlaw, it's evil."

Play video content

Reynolds adds, "If he wants help in quitting, I'll be happy to be his personal coach."