Gallagher Suffers Heart Attack -- Hospitalized in Texas

Watermelon-smashing comedy legend Gallagher is hospitalized in Texas tonight ... after suffering a heart attack at a bar moments before he was supposed to perform ... TMZ has learned.

65-year-old Gallagher was preparing to go on stage, when he began to experience intense chest pains. The owner of the club called 911 ... and emergency personnel responded to the scene and transported Gallagher to a nearby hospital.

Gallagher's manager tells TMZ ... the comic suffered a "mild to serious" heart attack and will remain in the hospital for the next couple of days while doctors try to figure out what's wrong with his ticker.