WATCH: Crowd sings ‘Happy Birthday’ to Trump at US Army’s 250th anniversary parade

As President Donald Trump closed his remarks and invited first lady Melania Trump to the dais along U.S. 50 in downtown Washington, the assembled crowd of thousands began an impromptu rendition of "Happy Birthday."

The president, who turned 79 Saturday, smiled and paused as the singing gave way to a young Army officer approaching with a folded U.S. flag.

The officer presented Trump the flag on behalf of the U.S. Army Golden Knights and said it had flown over the Capitol earlier in the day to commemorate the nation’s oldest military branch.

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Trump had received blowback, mostly from the left, for holding the Army commemoration with critics pointing out it also fell on his birthday.

"I’m horrified by the parade, because this is not about honoring our military," Sen. Adam Schiff, D-Calif., a frequent Trump critic, said on a recent podcast. "It’s a military vanity show for a president who’s long wanted to have tanks rolling down the streets of our nation’s capital."

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Rep. Steve Cohen, D-Tenn., also recently drafted the HAPPY BIRTHDAY Budget Act, which would prohibit taxpayer dollars for such celebrations.

During the parade, Vice President JD Vance noted that Saturday was his and second lady Usha Vance’s wedding anniversary. It is also Flag Day, the date in 1777 when the Continental Congress adopted Betsy Ross’ Stars and Stripes flag.