The march had been planned openly. The Ponce branch of the Cadets of the Republic, the Nationalist Party's youth organization, intended to parade through downtown under the command of Tomas Lopez de Victoria. Ponce mayor Jose Tormos Diego had granted the necessary permits days earlier. But on the morning of the march, Governor Blanton Winship -- appointed by Washington to govern Puerto Rico -- ordered the Insular Police to revoke the permits and block the procession. By the time the marchers assembled, armed police had surrounded them on multiple sides. The Nationalists, aware of the revocation but determined to exercise what they considered their constitutional right to peaceful assembly, decided to proceed.

As the band struck up La Borinquena, Puerto Rico's anthem, and the marchers began to move, the police opened fire. The shooting continued for approximately fifteen minutes. Witnesses and investigators later documented that police fired into the crowd from multiple directions, trapping marchers in a crossfire with no avenue of escape. Among the 19 people killed were men, women, and a young girl. Over 200 civilians were wounded, including bystanders who had simply been on the street that Palm Sunday morning. Two police officers were also killed, though investigators concluded they were likely hit by their own colleagues' fire. Not a single firearm was found among the marchers or the dead.

Two investigations followed. The first, conducted by the Insular government itself, produced inconclusive results. The second carried far more weight. The American Civil Liberties Union sent a commission led by Arthur Garfield Hays, a prominent civil liberties attorney, along with several respected Puerto Rican citizens including Fulgencio Pinero and Emilio Belaval. Their findings were unambiguous: what happened on March 21 was a massacre. The Hays Commission placed blame squarely on Governor Winship and his administration's repressive policies toward the Nationalist movement. The commission questioned why police had fired directly into the crowd of civilians rather than at the Cadets, and documented systematic civil rights violations under Winship's governance.

The massacre drew international attention and sharp criticism from members of the U.S. Congress, most notably Representative Vito Marcantonio of New York, who delivered a blistering speech on the House floor in August 1939 detailing what he called five years of tyranny in Puerto Rico. The political pressure mounted until President Franklin D. Roosevelt removed Winship as governor that same year. But for the families of the dead and wounded, accountability remained elusive. No police officer was ever prosecuted for the killings. The event deepened the divide between those who sought Puerto Rican independence and the colonial administration, fueling activism that would continue through the Nationalist revolts of the 1950s.

Today the corner of Marina and Aurora streets, where the marchers fell, houses the Museo de la Masacre de Ponce, operated by the Institute of Puerto Rican Culture. The museum preserves photographs, documents, and artifacts from that Palm Sunday, including a section dedicated to Pedro Albizu Campos. A nephew of one of the victims, playwright Roberto Ramos Perea, wrote the play Revolucion en el Infierno, which was adapted into a television film in 2004, telling the story through the life of his uncle Ulpiano Perea. The massacre remains one of the most significant events in Puerto Rican political history -- a day when unarmed people exercising their right to march were gunned down by the forces meant to protect them, and when the world was forced to see the human cost of colonial rule.