The guerrilla group of slain Palestinian leader Zuhair Mohsen reportedly threatened today to assassinate Egyptian President Anwar Sadat in revenge for Mohsen's death, while other followers forced the Moslem sector of Beirut to shut down in mourning.

The military security chief of Saiqa, the Syrian-controlled guerrilla group that Mohsen headed, was quoted by the official Kuwaiti News Agency as charging that Egypt's secret service and the Israeli intelligence agency, Mossad, had acted jointly to assassinate Mohsen. The 43-year-old guerrilla leader died in a hospital in the French Riviera city of Nice yesterday after being shot in the head while entering his rented apartment in nearby Cannes early Wednesday.

"Saiqa will strike back," the group's security chief told the news agency in an interview in Kuwait. "Egyptian officials, including President Sadat, are within our fighters' reach."

Identified only by the code name Abu Thayer, the Saiqa official was quoted as asserting that the group has "documents proving" Egyptian and Israeli involvement in the assassination of Mohsen, who was also the Palestine Liberation Organization's chief of military operations.

Egypt has blamed Saiqa for three attacks since Sadat signed a peace treaty with Israel March 26: a bombing of the Egyptian Embassy in Nicosia, Cyprus; a letter-bomb explosion at the Cairo central post office, and a hostage-taking assault on the Egyptian embassy in Ankara.

The three attacks were claimed by a group calling itself the "Eagles of the Palestinian Revolution," which has been described as Saiqa's terrorist arm. Saiqa in turn is said to function as a branch of the Syrian armed forces.

Saiqa gunmen, firing bursts from their Soviet-made AK47 automatic rifles, burned tires in the streets of predominantly Moslem West Beirut and forced businesses to close in tribute to Mohsen.

The PLO, which is made up of eight guerrila factions including Saiqa, issued a joint call with its Moslem Lebanese leftist allies for a general strike in their sectors of Lebanon Saturday, the day that Mohsen's body is to be flown to Damascus. He is to be buried Sunday.

Meanwhile, in other Middle East developments:

Doctors reported further improvement in the condition of Israeli Prime Minister Menachem Begin, who was hospitalized a week ago because of a minor obstruction in an artery to his brain.

Lebanon's state radio said Israel fired missiles at a string of villages used as outposts by Syrian peacekeeping forces in southeastern Lebanon. Israeli military sources said they had no knowledge of such action.

Egypt's Middle East News Agency reported that a "communist" coup attempt against new Iraqi President Saddam Hussein was foiled Monday. The report from Beirut quoted Arab diplomatic sources as saying five members of Iraq's ruling Baath Party and some army officers were arrested following the unsuccessful coup.