Supporters of President Donald Trump crashed New York Mayor Bill de Blasio’s press conference touting the city’s “Green New Deal” inside the lobby of Trump Tower on Monday morning, holding signs with insults directed at the far-left Democrat.

Speaking at a podium steps from the spot where President Donald Trump announced his own presidential run in 2015, the Democrat said he had come to Trump Tower to promote a new city air pollution law requiring skyscraper owners to cut greenhouse gas emissions or face fines.

De Blasio said Trump’s buildings in the city could face a combined $2.1 million in annual fines if they don’t reduce emissions by 2030.

“President Trump, you’re on notice. Your polluting buildings are part of the problem,” he claimed.

However, de Blasio’s speech was all-but drowned out by a throng of Trump supporters who demonstrated behind him on the building’s escalators, booing, whistling, chanting and holding up signs with messages including “Failed mayor” and “Worst mayor ever.” Footage of the heckling was shared to social media.

Trump suppoters protest as New York City Mayor Bill de Blasio speaks inside Trump Tower about the Green New Deal, serving notice to US President Donald Trump demanding more energy-efficient buildings, including Trump Tower, May 13, 2019 in New York. (Photo by Don Emmert /AFP/Getty) 

Loud music also played over the building’s sound system for part of the event. De Blasio had to shout to be heard.

“It’s so nice of them to serenade us at Trump Tower. Clearly, they are uncomfortable with the truth,” he said.

NEW YORK, NY – MAY 13: Protesters gather as Mayor Bill De Blasio holds a Green New Deal rally At Trump Tower in New York City on May 13, 2019. Mayor de Blasio recently unveiled his Green New Deal to reduce carbon emissions in New York City. (Photo by Yana Paskova/Getty Images)

Under a deal that President Trump struck with the city decades ago, Trump Tower’s lobby is considered a public space for much of each day.

New York City’s $14 billion “Green New Deal” aims to reduce emissions by roughly 30 percent. The plan will ban new glass buildings deemed “inefficient” and strive for carbon neutrality by 2050.

The Associated Press contributed to this report.