Yasser Arafat apparently has assented to rebel demands for sharp restrictions on his powers as chairman of the Palestine Liberation Organization, according to reports today from Tunis and Damascus.

The concessions, in exchange for saving his position, surfaced as a six-member mediation delegation of the PLO Executive Committee met with Syrian officials in an effort to quell the two-month old rebellion within Fatah, the PLO's largest faction and previously Arafat's main base of support.

According to the reports, Arafat has agreed to share leadership of the organization pending a congress of all members. That is a rebel demand that Arafat loyalists had said he would never accept.

The PLO chairman has also agreed to renounce formally President Reagan's Middle East peace initiative and thereby extinguish any flickering hope that Arafat would give Jordan's King Hussein the mandate he says he needs to negotiate on behalf of Palestinians.

The Associated Press quoted an unidentified PLO official in Tunis as saying the mediation committee had proposed "an immediate and firm cease-fire in the Bekaa Valley and a withdrawal of Fatah forces loyal to Arafat from the area" to disengage them from the rebels.

Arafat's acquiescence to the rebels would appear to move the PLO away from his more moderate position and toward the rebels' hard-line stance, adding to the complexity of Secretary of State George P. Shultz's quest for the withdrawal of PLO, Syrian and Israeli forces from Lebanon.

Col. Said Musa, the rebel leader also known as Abu Musa, said in an interview published in The Guardian newspaper in London today that the rebels are committed to a policy of "no reconciliation, no recognition and no negotiation" with Israel.

"We know that Arafat wants to leave Lebanon," Abu Musa said, suggesting that the PLO chairman is prepared to agree to dispersing fighters in Arab countries as he did last summer in the evacuation of Beirut.

"There can be no armed struggle from Algeria or Yemen," Abu Musa said. "If Arafat continued to send out fighters, from where could we say 'no ' to him? No one would listen to us."

Speaking at his headquarters in the Bekaa Valley, Abu Musa said he does not believe in a Palestinian state on the West Bank, except as a halfway stage to a united Palestine.

Asked if he envisaged expelling Jews from an eventual Palestinian state, he said: "No, why should we? Not the Jews who were present in Palestine, but those who emigrated to Palestine with the rise of Israel are not Palestinians. . . . They must go back to the countries they came from. They are nationals of Europe--of France, of Britain."

Abu Musa said he is opposed to Arafat's policies of the last 10 years.

"Arafat is absolutely individualist," he said. "Often, on fundamental issues, he does not even consult members of the PLO Executive Committee."

Rebels and loyalists agreed to the third cease-fire in four days of on-again, off-again fighting in the Bekaa Valley, where PLO guerrillas behind Syrian front lines are separated from Israeli forces. The two previous cease-fires came as a result of heavy pressure from Palestinian refugees and Lebanese civilians who objected to having their homes turned into battlefields.

The six-man PLO team in Damascus did not appear to have as much success, however, in ending the rift between Arafat and Syrian authorities, who deported him last month after he said publicly that rebels were being aided by Syrian tanks and soldiers.

A close aide to Arafat said in Tunis yesterday that word had been passed that Arafat would be allowed to return to Syria only after he publicly recanted his accusations, and after the demands of the dissidents are put into effect.