US President Donald Trump was booed on Thursday while attending a New York Yankees baseball game on the 24th anniversary of the Sept. 11 terrorist attacks.

Before nearly 41,000 fans, chants of “USA” and Trump’s name mixed with boos that grew louder each time he appeared on the big screen during the national anthem.

Authorities placed security glass around the private box suite on the third-base side of Yankee Stadium, where Trump watched the game alongside Yankees President Randy Levine.

Before the game, Trump met with players in the Yankees’ clubhouse, shaking hands with star outfielder Aaron Judge, manager Aaron Boone, and several others.

The team also wore hats bearing “FDNY” and “NYPD” in tribute to first responders on the anniversary of the Sept. 11 attacks.

Security measures were further tightened after conservative activist and Trump ally Charlie Kirk was assassinated in Utah on Wednesday.