Leaders of the hitherto secret organization Harakat Tahrir, among them its founder Muhammad Bassiri, called a demonstration to hand a petition calling for independence and fair treatment for Sahrawis to the Spanish governor-general of the colony, General José María Pérez de Lema y Tejero. They were allowed to read out the petition, but as the demonstration was dispersing, police moved in to arrest its leaders. Demonstrators responded by pushing the policemen away and when the police charged with batons, stone-throwing erupted. The Spanish authorities called in the Spanish Foreign Legion who opened fire on the demonstrators, killing at least eleven and wounding scores. Hundreds of people were arrested.
In the days following the massacre, Harakat Tahrir activists, Bassiri among them, were hunted down by Spanish security forces. Bassiri disappeared in jail.
The suppression of the Zemla demonstration pushed the Western Saharan anti-colonial movement into embracing armed struggle. The militant nationalist organization Polisario Front was formed three years later.
June 17 is now commemorated by Polisario supporters in Tindouf, Algeria, and has been used as a reference day for protests in Western Sahara.