TUNIS, NOV. 7 (SATURDAY) -- Tunisian President Habib Bourguiba, who ruled his country for 30 years, was deposed today by his second-in-command, Tunis radio said.

Prime Minister Zine Abidine Ben Ali said in a statement on the state-run radio that he had taken over the reigns of power from the 84-year-old president under a constitutional provision dealing with the "incapacity for medical reasons" of the president.

Stressing that his takeover was in accordance with the constitution, Ben Ali said he had now become president and head of the armed forces.

Bourguiba, long one of the United States' staunchest allies in the Arab world, has been known to be ill for months. His current health condition was not immediately known.

It was uncertain whether Ben Ali's decision had been taken solely by the him or whether it resulted from consultation with the rest of the government.

Bourguiba appointed Ben Ali as prime minister last month, following the trial of 90 Moslem 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.

Ben Ali, a retired general who once received intelligence training in the United States, was the first former military man to have held the post of prime minister. During a term as Interior Minister, in charge of the country's security police, he gained a reputation as a tough-minded, conservative administrator.

Tunisia became independent from France in 1956 and Bourguiba had led the pro-Western government in Tunis since July 25, 1957.

Bourguiba 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.

Since the 1970s, Bourguiba's advancing age, combined with economic difficulties and growing unpopularity of the ruling Destourian Socialist Party, have raised concerns among political analysts and diplomats about the stability of the country.

Reports from Tunis described the capital of 600,000 people as calm early today. Following Ben Ali's statement, the state-run radio continued its regular programming.