The founder of the radical Popular Front for the Liberation of Palestine (PFLP), George Habash, has died in Jordan, associates say.
Habash died of a heart attack, one of his former colleagues said. He was around 80 years old.
Palestinian President Mahmoud Abbas has called three days of national mourning.
Under Habash's leadership the PFLP became known as one of the most radical Palestinian militant groups, notorious for plane hijackings.
In September 1970 it captured four airliners, later blowing them up in front of the world's cameras, once the passengers had been removed.
The group also fired at Israeli aircraft and their crew, carried out bombings, and took hostages including, on one occasion, Opec leaders meeting in Vienna.
George Habash was born in Lydda, in British-ruled Palestine - which is now Lod in Israel - and was forced to flee when war broke out in 1948.
He was a Christian, and became a medical student, who espoused the cause of secular Arab nationalism.
1968 Hijacks Israeli plane in first major operation
1970 Hijacks four Western airliners; blows them up after passengers removed
1970 Swissair flight SR330 bombed in mid-flight; 47 dead
1972 Japanese leftists carry out massacre at Lod Airport, Tel Aviv, for PFLP; 26 dead
1975 Carlos the Jackal leads PFLP hijacking of Opec meeting in Vienna; hostages flown round Middle East
1976 Air France flight from Tel Aviv hijacked and diverted to Entebbe, Uganda; Israeli paratroopers mount raid, freeing most hostages
2001 Israeli Minister for Tourism Rehavam Zeevi assassinated
He was a founding member, in 1951, of the Arab Nationalist Movement, which later merged with other groups to become the PFLP.
The group designated itself a Marxist-Leninist movement and became the second largest partner in the Palestinian Liberation Organisation.
It carried out more and more audacious attacks in the 1960s, throwing the Palestinian issue into the international spotlight.
It opposed Arab-Israeli peace negotiations and any talk of a two-state solution - Habash often criticised PLO leader Yasser Arafat for negotiating with Israel.
Throughout his life Habash supported the use of violence against Israel.
One of the most deadly PFLP attacks was the gunning down of 27 people at Israel's Lod airport in May 1972.
Forty-seven people were killed when a Swissair jet was bombed in 1970.
Israel tried for years to capture Habash, even intercepting a commercial airliner in 1973 and forcing it to land, mistakenly believing he was on board.
Habash stepped down as leader of the PFLP in 2000, by which time the group had been marginalised by the growing power of Islamist movements like Hamas.
Mr Abbas called Habash a "historic leader" and ordered Palestinian flags to fly at half-mast.
The current deputy secretary-general of the PFLP, Abdel Raheem Mallouh, called Habash a "distinguished leader... who struggled for more than 60 years without a stop for the rights and the interests of his people".
He was married, with two daughters.
His wife, Hilda, said: "We will all carry the banner of Habash and the Arab nation he dearly cherished. Habash always believed Palestine would be liberated."
"He lived for his people and died for his people," she told Reuters.