Motley Crue guitarist Mick Mars was knocked to ground by an overzealous fan last night after the guy bum-rushed the stage in the middle of a concert ... and sent the 62-year-old birthday boy flying.

The show took place at Spectra Place in Estevan, Saskatchewan, Canada on Saturday ... which just so happened to be Mars' bday.

The band was just finishing the song "Primal Scream" when the guy bolted across the stage, knocking over Mars and then making a run at lead singer Vince Neil ... before security stepped in and hauled the guy away.

The group left the stage briefly to make sure Mars was okay to continue, which he was.

The incident was particularly harrowing because Mars suffers from a painful medical condition called ankylosing spondylitis, which slowly fuses the bones together.