Taylor Swift -- Felony Assault Onstage ... Fan Busted for Brawl (VIDEO)

Taylor Swift never broke stride as a crazed fan jumped onstage and went toe-to-toe with her security force in a fight so nasty ... bones were broken.

The intense brawl went down Saturday night at San Diego's Petco Park, and law enforcement sources tell us 26-year-old Christian Ewing snuck into the concert -- repeatedly evading security guards trying to toss him.

We're told Ewing eventually charged onto the stage just as Taylor was runway strutting during "Style." Check out the video ... 4 security guards immediately pounced on him, but he put up a hell of a fight.

The guards eventually dragged him away -- but one guard broke a rib. Ewing was booked for felony and misdemeanor battery, and obstruction. His bail is set at $35k.

Besides a quick glance ... Taylor was able to shake it off.