The rockers were playing at the Spectra Place in Saskatchewan, Canada, when an overzealous audience member invaded the stage and pushed over the guitarist, who was celebrating his 62nd birthday.
The unidentified man then charged toward frontman Vince Neil but security stepped in and escorted him away.
The band then headed backstage for a short break to look after Mars, who suffers from a painful inflammatory condition called ankylosing spondylitis, before reemerging to wrap up the gig.
Mars later took to his Twitter.com page to assure fans he is Ok, writing, "Thank all of you for your concerns about me being knocked down last night... I'm alright... nothing broken. My body guard Rhyno got 2 busted ribs."