The mutiny broke out on 8 September 1936 among communist sailors in the Portuguese Navy's two newest warships moored in the estuary of the Tagus River near Lisbon.

However, the revolt failed and the convicted sailors were the first to be sent to the Tarrafal concentration camp established in the Cape Verde Islands to house political prisoners.

Several Spanish conservatives sought refuge in Portugal, where a right-wing dictatorship, the Estado Novo, established in 1926 and led by António de Oliveira Salazar, felt threatened by the change in power in Spain.

[2] The Portuguese government offered support to Franco's forces in defiance of a non-intervention agreement they had been pressured to sign by their ally, the United Kingdom.

The British government warned the Portuguese that they would not protect them from attacks by the Republican faction if Portugal continued to be involved in the war, making Salazar increasingly nervous about his position.

Through the Revolutionary Organization of the Fleet (Organização Revolucionária da Armada, ORA), which had grown in strength over the course of the early 1930s,[4] the party plotted a mutiny of several Portuguese Navy ships with intention of allowing them to sail to Spain to assist the Spanish government in the war.

By then the crews of Afonso de Albuquerque and Dão, two of the navy's most modern ships,[8] had already mutinied, forcing their officers below deck at gunpoint.

The sailors on Afonso de Albuquerque attempted to lure the Admiralty officers aboard, but the launch fled and the crew opened fire with machine guns.

[9] According to historian Glyn Stone, the revolt was "easily suppressed and remained an isolated incident" and did not pose a threat to Salazar.

[13] The German Ambassador to Portugal, Oswald von Hoyningen-Huene, reported that "it is even said that Salazar...provoked the dramatic development, or at the very least allowed matters to run their usual course.

[7][10] On 9 September, Salazar issued an official statement which depicted the Spanish Civil War as an international conflict and warned of the dangers of political contagion.