The Popular Front for the Liberation of Palestine announced yesterday it has expelled Dr. Wadi Haddad, the reputed mastermind of international terrorism.

A brief statement from the PFLP's headquarters here said Haddad was ousted last year but the announcement was necessitated by the hijacking last month of a Lufthansa jet to Mogadishu, Somalia, by four terrorists. The statement indicated the four belonged to Haddad's dissident group called the PFLP-Special Operations.

The onetime pediatrician, who appears to command the allegiance of japan's Red Army guerrillas and the reminants of West Germany's Baader-Meinhof gang, was apparently expelled following differences of opinion with the PFLP's leader. Dr. George Habash, over the use of hijacking as a terrorist tactic.

Habash insists that hijacking was used by the group only to gain attention and after a multiple hijack staged in 1970 it served no more useful purpose. Apparently Haddad still favored the tactic.

The two physicians-turned-guerrillas held the top posts for years in te PFLP, the Marxist maverick within Yasser Arafat's guerrilla movement. Habash served as the leader, top decision maker and ideological theoretician, while Haddad headed PFLP's foreign operations bureau, which staged some of the world's most spectacular terror operations.