The students were protesting the May 4 Kent State shootings and the Vietnam War, following the April 30 announcement by President Richard Nixon of the U.S. invasion of neutral Cambodia.

On May 4, 1970, thirteen students were shot, four of them fatally, at Kent State University in Ohio by National Guardsmen as they demonstrated against the U.S. involvement in the Vietnam War and U.S. incursions into neutral Cambodia.

Peter J. Brennan, president of the Building and Construction Trades Council of Greater New York, was a strong supporter of Nixon's policy of Vietnamization and ending U.S. involvement in the war.

By late morning, after some high school students, teachers and others joined, more than a thousand protesters were gathered in the street in front of Federal Hall and on the steps around George Washington's statue.

When one mayoral aide lowered the flag back down to half-mast, hundreds of construction workers stormed the area around City Hall, leading to a melee similar to the one on Wall Street the hour prior.

Deputy Mayor Richard Aurelio, fearing the building would be overrun by the mob, ordered city workers to raise the flag back to full mast.

Several groups of construction workers stormed the newly-built main building at Pace University, smashing lobby windows and beating up students and professors, including with tools.

The most common victim was a "22-year-old white male collegian" and the worst injuries were to the "half-dozen young men beaten unconscious," but about one in four of the injured were women.

[17] During a press conference that evening, President Nixon tried to defuse the situation before tens of thousands of students arrived in Washington, D.C. for a scheduled protest rally the next day.

Before dawn, the next morning, Nixon told some protesters that, "I understand just how you feel" and defended the recent troop movements into Cambodia as aiding their goal of peace.

[21] NYPD leaders later accused Lindsay of "undermining the confidence of the public in its police department" by his statements,[22] and blamed their inaction on inadequate preparations and "inconsistent directives" in the past from the mayor's office.

[27] The construction workers and police were both mostly "white ethnics", lived in the same neighborhoods, and socialized in similar establishments; many were also veterans of World War II and Korea, and both were also disproportionately likely to have family and friends in Vietnam.

[28] On Sunday, May 10, White House Chief of Staff H. R. Haldeman wrote in his diary, "The college demonstrators have overplayed their hands, evidence is the blue-collar group rising up against them, and [the] president can mobilize them".

[33] On May 26, Brennan led a delegation of 22 union leaders, who represented more than 300,000 tradesmen, to meet with Nixon at the White House and presented him with several ceremonial hardhats and a flag pin.