In October 1908, the first nationalist movement was formed, Budi Utomo. On 10 September 1912, the first nationalist mass movement was formed: Sarekat Islam. By December 1912, Sarekat Islam had 93,000 members. The Dutch responded after the First World War with repressive measures. The nationalist leaders came from a small group of young professionals and students, some of whom had been educated in the Netherlands. In the post–World War I era, the Indonesian communists who were associated with the Third International started to usurp the nationalist movement. The repression of the nationalist movement led to many arrests, including Indonesia's first president, Sukarno (1901–70), who was imprisoned for political activities on 29 December 1929. Also arrested was Mohammad Hatta, first Vice-President of Indonesia. Additionally, Sutan Sjahrir, who later became the first Prime Minister of Indonesia, was arrested on this date.