Green Day's Anti-Trump Chant Was Shock to ABC

10:20 AM PT -- The anti-Trump chant was not pre-planned or withheld during rehearsals, it was a last minute decision by the band ... according to AMA sources.

Green Day kept its plans to blast President-elect Donald Trump under wraps, never giving ABC or American Music Awards producers any warning ... TMZ has learned.

Witnesses at AMA rehearsals tell us Green Day performed its new song, "Bang Bang" ... but never included the chant, "No Trump! No KKK! No fascist USA!” ... which they added during the live broadcast. The performance ended with a standing O. We're told ABC execs were completely off-guard.

It's unclear if Green Day knowingly kept the chant out of rehearsals, or if Billie Joe Armstrong made a last minute decision to include the political message.