Tens of thousands of followers of Ayatollah Ruhollah Khomeini demonstrated in Tehran today as Iranian jet fighters and helicopters flew low over the crowds in an apparent military show of support for the government of Prime Minister Shahpour Bakhtiar.

The demonstrations followed Khomeini's announcement yesterday that he has named Mehdi Bazargan, a prominent Iranian opposition figure, as prime minister of a provisional government Khomeini is establishing to rival Bakhtiar's official government.

Bakhtiar told the lower house of parliament today that he would remain in office until the next general election. In a clear reference to Khomeini's naming of a prime minister, Bakhtiar said, "I have nothing to do with governments that exist in the imagination of people and are more of a joke."

The demonstrations in Tehran today were largely peaceful, but the state radio reported three persons killed and 27 injured in antigovernment demonstrations in Zahedan, near the Pakistani border.

In other developments:

Foreign Minister Ahmad Mir-Fendereski said Iran has decided to withdraw from the Central Treaty Organization (CENTO), the U.S.-backed alliance that was formed in 1959 but has been largely inactive militarily in recent years. "The continuing presence of Iran in CENTO is inexplicable in the context of Iran's new foreign policy," Mir-Fendereski said.

About 400 more Americans were evacuated from Iran, reducing the number here to about 5,000. One American who left today, George Van Vladricken, said many had received "death threats and other abusive notes."