Beirut — The month-old mutiny within the Fatah wing of the PLO appears to be headed for a showdown between dissidents and supporters of chairman Yasser Arafat. The rebels, led by Col. Abu Musa, have extended their demands by calling for a provisional leadership to run Fatah until a ''new ruling structure'' is established. The mutineers want half of the seats on the 50-member central committee, as well as the appointment of Abu Musa as Mr. Arafat's deputy.
Such a change would in effect cost Mr. Arafat his dominance, even if he kept the top job. Since Fatah represents about 80 percent of the Palestine Liberation Organization, it would also dramatically alter his ability to control the eight-sided guerrilla movement, and push ahead with his moderate policy of leaning more toward diplomacy than confrontation with Israel.
The rise in the number of the mutineers was reflected in a statement from Mr. Arafat's deputy commander, Abu Jihad: ''Every time we solve a problem, they raise fresh ones.''
Last Friday Mr. Arafat launched a whirlwind tour of countries that have traditionally backed him in hopes of rallying sufficient international leverage to pressure Syria, and to a lesser degree Libya, to end their support for the rebels. So far he has been to Romania, Algeria, Saudi Arabia, and India.
In turn, Saudi Crown Prince Abdullah as well as Arab League Secretary-General Chadli Klibi went to Damascus to discuss the PLO split and the Syrian rejection of withdrawal of foreign forces from Lebanon.
Soviet help has also been elicited, after a Fatah delegation visited Moscow. Kremlin leader Yuri Andropov has sent messages to both Syrian President Hafez Assad and Mr. Arafat announcing his backing of ''a strong and unified position, based on the relationship between the PLO and Syria, as well as Palestinian unity under its legitimate leadership, headed by chairman Yasser Arafat,'' according to WAFA, the Palestinian news agency.
The Soviet intervention is significant since Syria is dependent on Moscow's military aid. It is also interesting because the rebels' demands are more in line with Moscow than are Mr. Arafat's policies. And the man heading the political side of the mutiny, Abu Saleh, is the leading pro-Moscow leftist within Fatah.
Saudi support has also been crucial, since the kingdom is a major aid donor to the Assad regime. King Fahd took two unusual steps during the Arafat visit Tuesday. He greeted him at the airport and went to visit him at the guest palace rather than having the PLO leader pay an official call on the monarch.
The Saudi press gave major play to King Fahd's welcoming remarks: ''We are fortunate to have with us today Yasser Arafat, the leader of the PLO on whom we pin great hopes.''
Yet so far there are no signs that the pressure tactics have worked on either the independent Syrians or the rebels. Indeed the strategy has mainly served to show Mr. Arafat as a desperate man on the verge of losing his 15-year hold on the guerrilla movement.
As he was jetting around at a frantic pace, a four-hour clash broke out over the weekend between his supporters and dissidents in Lebanon's eastern Bekaa Valley. Both sides had earlier pledged to use ''democratic means'' to resolve their disputes, so the brief battle underlined the growing polarization.
Meanwhile, three smaller PLO factions, all aligned with Syria and Libya, issued a communique in Damascus accusing Arafat of abandoning the armed struggle against Israel and criticizing his policy - indicating the mutiny is spreading.
Capt. Ahmed Jebril of the Popular Front for the Liberation of Palestine-General Command said, ''We do not accept any dialogue with Arafat unless he accepts first to carry out the requested reforms inside both Fatah and the PLO.''
In an apparent concession to one of the rebels' demands, Abu Jihad was quoted as announcing that guerrilla units dispersed to the other seven Arab states after the evacuation will soon be brought back to Syria and PLO frontline positions in eastern Lebanon.
Although diplomats have been evaluating the mutiny on the basis of the numbers of high level officers who have joined - now estimated at 24 - it is in fact the second generation of young guerrillas who are the key to the future of Arafat and the PLO. Their return may spark further problems.
The vast majority were not born in Palestine, have no experience in living with Jews as neighbors, and have witnessed only violence during the 35-year conflict. They are usually more militant in their outlook and tactics, and thus often sympathetic to the challenge of Arafat's attempt to achieve a negotiated settlement with Israel.