contract workers were being subjected to forced labor and unsatisfactory working conditions. During the Great Depression, worker exploitation worsened. Sporadic labor unrest and dissatisfaction continued well into the 20th century, culminating in an outbreak of riots in 1953 in which several hundred African laborers were killed in a clash with their Portuguese rulers. This "Batepá Massacre" remains a major event in the colonial history of the islands, and its anniversary is officially observed by the government. During the 1967–70 secession war from Nigeria (Nigerian Civil War), São Tomé served as the major base of operations for the Biafran airlift. The airlift was an international humanitarian relief effort (the largest civilian airlift to date) that transported food and medicine to eastern Nigeria. It is estimated to have saved more than a million lives. Movement towards independence By the late 1950s, when other emerging nations across the African Continent were demanding independence, a small group of São Toméans had formed the Movement for the Liberation of São Tomé and Príncipe (MLSTP), which eventually established its base in nearby Gabon. Picking up momentum in the 1960s, events moved quickly after the overthrow of the Caetano dictatorship in Portugal in April 1974. The new Portuguese regime was committed to the dissolution of its overseas colonies; in November 1974, their representatives met with the MLSTP in Algiers and worked out an agreement for the transfer of sovereignty. After a period of transitional government, São Tomé and Príncipe achieved independence on July 12, 1975, choosing as its first president the MLSTP Secretary General Manuel Pinto da Costa. Modern São Tomé and Príncipe In 1990, São Tomé became one of the first African countries to embrace democratic reform and changes to the constitution—the legalization of opposition political parties—led to elections in 1991 that were nonviolent, free, and transparent. Miguel Trovoada, a former prime minister who had been in exile since 1986, returned as an independent candidate and was elected president. Trovoada was re-elected in São Tomé's second multiparty presidential election in 1996. The Party of Democratic Convergence (PCD) toppled the MLSTP to take a majority of seats in the National Assembly, with the MLSTP becoming an important and vocal minority party. Municipal elections followed in late 1992, in which the MLSTP came back to win a majority of seats on five of seven regional councils. In early legislative elections in October 1994, the MLSTP won a plurality