Argentine workers campaign for human rights (Semana Roja), 1909

In the late nineteenth century, the Argentinian working class had gained greater self-identification during the economic crisis of 1898–1904, when labor strikes – an unknown phenomenon up to then – unleashed the rapid expansion of labor organizing and labor unions, and the national FORA (Regional Argentinian Workers' Federation) was created. In the first decade of the 20th century, union actions were met with extreme repression by the state, which proved incapable of responding through conciliation, leading to general strikes in 1902, 1904, and 1906. Although each strike had different objectives, all sought to end working-class oppression.

In 1902, the Argentinian government initiated its first major anti-labor policy through the passage of the Law of Residence of 1902, permitting the national executive power to expel foreigners who “compromised national security or disturbed public order,” understood to refer to any immigrant seeking to form a union. Given that, at the time, immigrants comprised the great majority of workers and leaders in Argentina, the law was considered a means to hamper union organization.

In the early twentieth century the proletariat labored under severely degrading conditions, enduring 11-hour workdays or longer, low pay, unhealthy workplaces, and tenement dwellings (“conventillos”). Unions were weak and the few members that existed tended to disperse at the first sign of repression. Politically, this period was characterized by rampant fraud, corruption, and a clientelist system controlled by large bourgeois families. There were no modern mass political parties, with the exception of the Socialists. The level of labor repression stimulated popular mobilizations and workers' protests to advance democratic freedoms that were recognized by the subordinated classes as a whole.

The most distinguishing feature of the Semana Roja was a tripartite division: anarchists—who evolved into communists—socialists, and revolutionary labor unionists. In 1909, at the time of the dispute, anarchists – who controlled the FORA (Regional Argentinian Workers' Federation) – were the dominant power, while the revolutionary labor unionists under the General Workers' Union (UGT), a socialist offshoot, were a significant presence. The rising influence of the UGT and revolutionary labor unionism during the Semana Roja further contributed to the decline of the anarchists after the repression of 1910.

May Day celebrations on May 1st, 1909 by the three political tendencies of the working class (the anarchist, socialist and revolutionary labor unionist groups) triggered the beginning of the Semana Roja. FORA leaders had called for a meeting of mass protest against the perceived governmental limitation of democratic liberties (such as freedom of expression, the right of assembly and decent labor conditions) and against repressive laws and regulations. Demonstrators gathered in Plaza Lorea in Buenos Aires (now Plaza de los Dos Congresos). They planned to march to another square, but as the protesters were about to begin their procession to the next destination, they were attacked by the police, who opened fire and killed ten people, injuring another seventy. The crowd scattered and ambulances rushed to remove bodies and the injured, and firefighters hosed the blood off the streets.

Consequently, the socialist May Day rally originally planned for the Plaza Constitución on the south side of Buenos Aires redirected its course and transformed into a protest action. The three political orientations (anarchists, socialists and revolutionary labor unionists) created a committee and declared a general strike, while the government of Roque Sáenz Peña launched an assault, calling out more policemen and military troops, arresting hundreds of people, and closing all workers' meeting places. The strike leaders demanded the reopening of meeting places, the liberation of prisoners, and the repeal of the Municipal Penalty Code, which forced workers' unions to accept a series of government controls that virtually eliminated the right to strike. Many workers also sought the resignation of the police chief, Ramón Falcón – widely believed responsible for the massacres – but strike organizers did not make the demand due to ideological differences. Whereas socialists vigorously demanded the police chief's resignation, anarchists and revolutionary labor unionists considered that the demand acknowledged the legitimacy of the bourgeois state, with which they refused to negotiate.

In the following days, between 200,000 and 300,000 workers of the city's working population of 500,000 demonstrated in the streets of Buenos Aires against the massacre. In response, the government mobilized the city's police force, arming it with war ammunition, calling for 5,000 army soldiers, two regiments of artillery, three cavalry regiments, six infantry battalions, two engineer battalions and 1,500 students from the military academy. The Government House (Casa Rosada) was protected by grenadiers.

The burial of protest victims brought out 50,000 to 80,000 people, the largest mass gathering ever before 1909 in Buenos Aires. Police attacked mourners on their way to the cemetery and 70 people were injured and 120 arrested. On the fourth day of the strike, a prominent socialist leader gave a speech emphasizing the campaign’s goals and the need for popular support and cohesion. The people that had gathered to listen to the speech were forced by the police to disperse, one person dying and several receiving injuries in the process. On the fifth and sixth days, the city was paralyzed. In response to the mass unrest, the government seemed willing to relent and to revoke the penalty code. On the seventh day of the strike, the leaders of the main political groups opened negotiations with the government. Benita Vilanueva, the president of the Senate, served as mediator.

Paradoxically, the socialists, who were the least militant of the three groups, demanded that Falcón's resignation be a condition of negotiations. Backed by popular sentiment that sought Falcón's dismissal, the socialists refused to enter into negotiations unless the government opened a discussion to include the dismissal of Falcón. But the anarchists and radical labor unionists, refusing to acknowledge the legitimacy of the bourgeois state, agreed to negotiate a settlement. After the government agreed to free prisoners and allow workers' meeting places to reopen, the strike was ended.

The campaigners’ analysis of the outcome differed according to their viewpoints: the socialists considered the strike a failure because the police chief had not resigned; the anarchists held ambiguous views; and the labor unionists called the strike a total success.

The Law of Residence remained in force until 1965 and more general strikes against it were organized in the years before 1930. Partly as a response to the Semana Roja of 1909, a second repressive law, the Law of Social Defense, was passed in 1910, permitting the government to incarcerate any worker who “disturbed public order” in a prison intended for serious criminals near Ushuaia in Tierra del Fuego, the extreme south of Argentina. The struggle of the Argentine working class continued into the 50s and 60s, supporting political developments such as Peronism.