(CNN) President Donald Trump put aside partisanship to honor the service and sacrifice of the nation's veterans , participating in the centennial New York City Veterans Day Parade Monday.

"Today we come together as one nation to salute the veterans of the United States Armed Forces, the greatest warriors to ever walk the face of the earth. Our veterans risked everything for us, now it is our duty to serve and protect them," he said during remarks, calling US veterans "America's greatest living heroes."

The parade is a "non-partisan, non-political event, and may not be used as a platform for any other purpose or political agenda," according to the event's website . New York Mayor and former 2020 Democratic presidential candidate Bill de Blasio was seated in the front row as Trump spoke, briefly recognizing and thanking him. However, there were audible protests as the President spoke, chants of "traitor" and "lock him up" heard as Trump spoke in Madison Square Park.

Joined by first lady Melania Trump, the President participated in a moment of silence and wreath laying in addition to his remarks at the opening ceremony.

Though presidents have historically always been invited to attend the parade, Trump was the first sitting president to accept the invitation, according to United War Veterans Council chairman Douglas McGowan. McGowan said in a press release that Trump "made a substantial donation in the mid 1990's to help the New York City Veterans Day Parade recover from years of neglect and rebuild into the institution it is today."

Trump, who did not serve in the US military himself, has a history of denigrating some veterans, including his frequent criticism of the late Sen. John McCain, whom he said was "not a war hero" because he was captured, and a contentious back and forth with Khizr and Ghazala Khan, a Gold Star family, during the 2016 election.

On Monday, Trump said, "To every Gold Star family, we will stand by your side forever."

His attendance at the parade was criticized by some veterans, including Paul Rieckhoff, Army veteran and founder and executive director of the Iraq and Afghanistan Veterans of America, who planned to boycott the event.

"Veterans Day is usually about unity. It's not about politics. And right now, we have the most divisive president in my lifetime, so by showing up, he is politicizing it. Now some are protesting, some are coming out to support him. It's immediately made this entire event about him," Rieckhoff said on CNN's "New Day," saying Trump's presence "reeks of politics."

"He could very easily go to Arlington (National Cemetery), he could go somewhere else, we could avoid all of this controversy if he just went somewhere else," Rieckhoff added.

Though he spent part of his morning railing on Twitter against House Intelligence Chairman Adam Schiff in the days before public hearings in the House impeachment inquiry begin, Trump's remarks were focused on veterans and US history, with a reference to the recent killing of ISIS leader Abu Bakr al-Baghdadi.

"Thanks to American warriors, al-Baghdadi is dead. His second in charge is dead. We have our eyes on number three. His reign of terror is over and our enemies are running very, very scared, thank you," he said.

The parade, per the United War Veterans Council, has more than 25,000 participants, who will travel 1.2 miles north on Fifth Avenue. Trump said he was "profoundly moved" to have veterans of World War II present in the audience for the ceremony, and recognized the veterans of other wars also on site.

Trump will remain in New York overnight Monday, delivering remarks to the Economic Club of New York City Tuesday before returning to Washington.