The May Days, a series of street battles fought between Republican factions of the Spanish Civil War, began on this day in 1937 when 200 police officers attempted to seize control of an anarchist press building. The May Day conflicts were the culmination of strife between various left factions within the Spanish Civil War.

On May 3rd, a body of 200 police officers went to the Telefónica central exchange and confronted the censorship department on the second floor, intending to take control of the building.

The anarchists, who were occupying the building legally, opened fire from the second floor of the building. Eventually, they surrendered with a cease fire agreement. A crowd gathered, at first believing that the anarchists had captured the head of the police.

The Workers' Party of Marxist Unification (POUM), Friends of Durruti Group, Bolshevik-Leninists, and Libertarian Youth all took firing positions and began taking out previously hidden weapons and building barricades. From this skirmish, battles began in different parts of the city.

Several hundred barricades were built. Police units occupied roofs and church towers. The fighting continued for several days and Assault Guards (state police and paramilitary units) in Barcelona and many other towns proceeded to disarm and arrest members of the CNT, FAI, Libertarian Youth, and POUM that had taken part in the riots.

Contemporary press estimated the casualties at 500 dead and 1,000 injured.