TMZ says that Gallagher was in the middle of his famous "sledge-o-matic" routine, in which he smashes fruits and vegetables with a giant sledgehammer, when he suddenly fell backwards.

Sources say "his eyes were closed, he was shaking" and that it could have been a heart attack or a stroke.

There's nothing official on his condition, but he was responsive when the EMTs arrived.  Coincidentally, Rochester is the home of Mayo Clinic, and is nicknamed "Med City."  So, he should be getting state of the art medical attention.

You can find some old footage of Gallagher's Sledge-O-Matic, here.  WARNING: There's some UNCENSORED profanity in this clip: