President Habib Bourguiba was deposed Saturday by his second-in-command, Prime Minister Zine Abidine Ben Ali, who named himself president of the North African nation.
In a statement read on the state-run Tunis radio, Ben Ali said the 84-year-old Bourguiba was deposed for "incompetence." Ben Ali said that based on his "faith in a medical report" he was removing Bourguiba from office "under Article 57" of the country's constitution.
Ben Ali said Article 57 deals with a president being unable to carry out his functions due to "incapacity for medical reasons."
Bourguiba has been known to be ill for months. His current health condition was not known immediately.
Stressing that his decision to take over the presidency was in accordance with the constitution, Ben Ali said he had now become president and head of the armed forces. It was uncertain whether Ben Ali's decision had been taken solely by him or whether it resulted from consultation with the rest of the government.
Bourguiba named Ben Ali prime minister Oct. 2, replacing Rashid Sfar. He had previously been interior minister.
He was appointed shortly after the trial of 90 Muslim militants accused of trying to topple the government and install a pro-Iranian regime in Tunis. Seven militants were sentenced to death, including five in absentia.
Reports from Tunis described the capital of 600,000 people as calm early Saturday.
Following Ben Ali's statement, the state-run radio continued its regular programming.
Bourguiba's Destourian Socialist Party has been the ruling party in Tunisia, a former French protectorate, since it gained independence from France in 1956.
Bourguiba had been president since 1957, when the Tunisian National Assembly voted to abolish the monarchy and establish a republic. He ran unopposed in the country's first election under a new constitution in 1959.
He had been Tunisia's sole leader since. He won four consecutive five-year presidential mandates through elections, and in 1975 the Tunisian Parliament amended the constitution to make him president for life.
According to the 1959 constitution, the president of the republic is chief of state, elected for a five-year term and permitted unlimited reelection.
Tunisia is North Africa's smallest country with 7.4 million people.