A leader of the Palestine Liberation Organization's dominant Fatah group has joined with four PLO member-organizations in denouncing the part of the Arab League peace plan adopted at Fez, Morocco, that offers implicit recognition to Israel.

The Fatah leader was Nimr Saleh, better known as Abu Saleh, a member of the PLO executive committee, who voiced his opposition Thursday in a press conference in Damascus, Syria, Reuter reported.

Saleh is known by Washington analysts as a pro-Soviet radical within Fatah. His opposition was seen here yesterday as reflecting his personal views rather than the official attitude of Fatah, which Yasser Arafat heads in addition to his position as leader of the umbrella PLO group.

The four Palestinian guerrilla organizations were identified as the Popular Front for the Liberation of Palestine, the Popular Front for the Liberation of Palestine-General Command, Saiqa and the Palestine Popular Struggle Front. The first two groups are Marxist-leaning, radical organizations within the PLO, and Saiqa is a Syrian-supported guerrilla group. The Palestine Popular Struggle Front is a small, little known group.

In a joint statement, the four groups said they objected to clause seven of the Fez proposals because it contained "recognition of the Zionist entity and of the legality of its usurpation of the Palestine land.

"The endorsement of this clause has opened the way for the Jordanian regime to exercise the role assigned to it by American imperialism . . . liquidating the Palestine cause and bypassing the PLO as the sole legitimate representative of the Palestine people."

The clause calls on the United Nations to guarantee the peace of all states in the Middle East.

Sources at the State Department expressed no surprise at the statement.

"It's not unexpected," said one specialist. "The extremists were bound to have their say sooner or later, but it doesn't affect the position of the PLO as a whole."

A spokesman for the Arab League, Nizar Jweidah, agreed, pointing out that two days before Arafat took part in the Fez conference he had received approval from the PLO executive committee to act with full freedom. The eight points of the Fez proposals were adopted unanimously Sept. 9 by the 20 members attending.