On March 21 , 1937 , a peaceful march was organized by the Nationalist Party , under Pedro Alibizu Campos , to commemorate the ending of slavery in Puerto Rico in 1873 by the governing Spanish National Assembly . The police , under the orders of General Blanton Winship , the US-appointed colonial Governor of Puerto Rico , opened fire at the peaceful Puerto Rican Nationalist Party parade , which is now known as the " Ponce massacre " : 20 unarmed people ( including two policemen ) were killed , with wounded persons ranging between 100 – 200 . This occurred because the head of the police force in Juana Díaz , Guillermo Soldevila , raised a whip and struck the chest of Tomás Lopez de Victoria , the captain of the cadet corps , and told him to stop the march . As a result , a police officer , Armando Martinez , ran from the corner in front of the Nationalist council and fired once into the air . This prompted many others to fire their arms
Ponce massacre
Ponce massacre
History of Puerto Rico