A fifth prime minister in the Iranian revolution

President Sayed Ali Khamenei appointed a new prime minister of Iran Monday and Ayatollah Ruhollah Khomeini attacked Amnesty International as ''Satanic'' for criticizing his regime's continuing executions.

Mr. Khamenei named Ali Akbar Velayati, a parliamentary aide, as the new prime minister to replace Muhammad Reza Mahdavi-Kani, who resigned last week. Mr. Velayati will be Iran's fifth prime minister since the Islamic republic was established in 1979 after the shah's overthrow.

Meanwhile Ayatollah Khomeini denounced Amnesty for its plea to halt Iran's executions and said the human rights group was part of a plot to prevent him from spreading his Islamic revolution to other countries.