George Habash, a longtime rival of Yasser Arafat who led the PLO's second-largest faction which pioneered airliner hijackings, died Saturday in Jordan. He was 81. "He had a severe heart attack and he died instantly," said Leila Khaled, a member of the Palestine National Council and a leading member of the Popular Front for the Liberation of Palestine, which Habash founded. Khaled spoke by telephone from Jordan Hospital in Amman, where she said Habash died at 8:15 p.m. Saturday. The PFLP's spokesman in Damascus, Maher al-Taher, also confirmed Habash's death. The former guerrilla leader had been living in Amman since 1992, al-Taher said. Habash, born to a Christian Arab family, was the leader of the more radical wing of the Palestine Liberation Organization and opposed Arab-Israeli peace talks. His group was the second-largest in the PLO after Fatah, the faction of Arafat and current Palestinian President Mahmoud Abbas. On hearing of Habash's death late Saturday, Palestinian Authority Chairman Mahmoud Abbas declared three-day mourning period and ordered flags to fly at half-mast. He called Habash a "historic leader" and said he would receive condolences at his office Sunday evening. Habash gained notoriety for promoting the Palestinian cause through terrorist attacks in the 1970s - including the hijacking of an Air France airliner to Entebbe, Uganda. In 1970, the PFLP hijacked four Western airliners over the United States, Europe, the Far East and the Persian Gulf. The aircraft were blown up in the Middle East after passengers and crews disembarked. The group also was responsible for gunning down 27 people at Israel's Lod airport in May 1972. Habash, an American-educated physician, launched the Popular Front in December 1967, six months after the Arabs lost the West Bank, Gaza Strip and Golan Heights to Israel. Habash opposed the interim peace agreements with Israel, in part because they did not require Israel to stop settlement construction. Throughout his life, he supported the use of violence against Israel, arguing that Israel would not make the concessions required for a peace agreement. However, since the early 1980s, he came to support the PLO platform, which calls for the establishment of a Palestinian state in the occupied territories and a "right of return" of Palestinian refugees. Habash frequently criticized Arafat, particular during his attempts to negotiate with Israel.