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John Legend is going to war with President Trump, the gloves are off and it's a full-on street fight ... the singer is calling the President a 'piece of s***' and a canker sore on this country.

The EGOT winner was leaving Peppermint nightclub Tuesday night in WeHo, where he was coming in hot on DT on the way out -- and it's truly something to behold. We know John isn't a fan of the Prez, but we've never seen him swinging quite like this.

He lashes out in a profanity-laced tirade against Trump, and it appears his recent comments about Baltimore and its representative in Congress, Elijah Cummings, is what set him off.

ICYMI ... Trump called Baltimore a "disgusting, rat and rodent infested mess" over the weekend, as well as a "dangerous & filthy place" -- criticizing Rep. Cummings for bashing the southern border sitch instead of tending to his own district. He's doubled down since then.

....As proven last week during a Congressional tour, the Border is clean, efficient & well run, just very crowded. Cumming District is a disgusting, rat and rodent infested mess. If he spent more time in Baltimore, maybe he could help clean up this very dangerous & filthy place — Donald J. Trump (@realDonaldTrump) July 27, 2019 @realDonaldTrump

Trump's latest perceived attacks against people of color seems to be the final straw for John -- who absolutely insists to our camera guy ... "We need to get him out of office."