On March 21, 1937 — Palm Sunday — Nationalists gathered in Ponce for a peaceful march marking the abolition of slavery and protesting Albizu's imprisonment. The municipal permit was revoked at the last minute on Governor Winship's orders; when the cadets stepped off anyway, unarmed, to the strains of La Borinqueña, police ringing the street opened fire for over ten minutes. Nineteen people were killed, including a seven-year-old girl and two policemen struck by their own crossfire, and some 200 were wounded, many shot in the back as they fled.
An independent investigation led by the ACLU's Arthur Garfield Hays found a deliberate massacre ordered by the governor — yet the only people prosecuted were surviving Nationalists. The Ponce Massacre remains the deadliest single act of political repression in twentieth-century U.S. territory.