The stage was set for a coup d'état, but the event that precipitated the coup was unplanned. On October 15, 1969, a bodyguard killed president Shermaarke while prime minister Igaal was out of the country. (The assassin, a member of a lineage said to have been badly treated by the president, was subsequently tried and executed by the revolutionary government.) Igaal returned to Mogadishu to arrange for the selection of a new president by the National Assembly. His choice was, like Shermaarke, a member of the Daarood clan-family (Igaal was an Isaaq). Government critics, particularly a group of army officers, saw no hope for improving the country's situation by this means. On October 21, 1969, when it became apparent that the assembly would support Igaal's choice, army units took over strategic points in Mogadishu and rounded up government officials and other prominent political figures. The police cooperated with the army.

Although not regarded as the author of the military takeover, army commander Major General Mahammad Siad Barre assumed leadership of the officers who deposed the civilian government. The new governing body, the Supreme Revolutionary Council (SRC), installed Siad Barre as its president. The SRC arrested and detained at the presidential palace leading members of the democratic regime, including Igaal. The SRC banned political parties, abolished the National Assembly, and suspended the constitution. The new regime's goals included an end to "tribalism, nepotism, corruption, and misrule." Existing treaties were to be honored, but national liberation movements and Somali unification were to be supported. The country was renamed the Somali Democratic Republic.