Donald Trump's arrival at Madison Square Garden was almost as anticipated as tipoff itself ... with POTUS instantly becoming part of the show when he stepped into the building for Game 3 of the NBA Finals.

Watch for yourself ... the MSG faithful made their feelings known ... turning Trump's appearance during the National Anthem into one of the loudest moments of the night so far.

And it seems the crowd's reaction wasn't lost on him as he let a smirk slip out while the crowd spoke ... or rather booed ... their piece.

Trump's attendance -- the first by any sitting president at an NBA Finals -- has been a major off-court storyline since he announced he would attend the Knicks' first NBA Finals game at MSG in 26 years, accepting an invite from team owner James Dolan.

How he would be received by fans sparked plenty of debate in the days leading up to tipoff ... not to mention getting into the Garden wasn't cheap, either ... with some shelling out thousands just for the worst seats in the house.

Former Knicks star Amar'e Stoudemire told TMZ Sports he believed the Commander-in-Chief deserved a standing ovation from the MSG crowd, while ex-NBA player Zach Randolph said Trump's presence shouldn't have any impact on the Knicks' focus because the team is locked in on chasing a title.

Not everyone saw it that way -- Rep. Alexandria Ocasio-Cortez questioned who could even afford tickets to the game in the first place ... suggesting the steep prices could influence the reaction to 47's attendance.

House Minority Leader Hakeem Jeffries was more blunt ... accusing Trump of having a habit of making major events about himself instead of letting fans enjoy the moment.

And ESPN personality Stephen A. Smith unloaded on "First Take" Monday morning, arguing Trump had no business attending the game and warning the visit would create a circus in Midtown Manhattan.

Game 3 brought way more than basketball ... Cardi B is set to rock halftime, Spike Lee fired up fans outside pregame, and Trump's arrival had security locked down tighter than a defense ... turning MSG into the center of the sports, celebrity and political worlds.