The Palestine Liberation Organization's military operations chief was critically wounded by an unidentified gunman in the French Riviera resort city of Cannes yesterday.

He was the latest victim in a series of attempted and successful assassinations against Middle East figures that goes back many years.

The assassination attempt left Zuhair Mohesen, 43, in what doctors called "hopeless" condition in a deep coma after brain surgery at a hospital in nearby Nice, According to news agency reports of the incident.

Mohsen, who also commands the Plo's pro-Syrian Saiqa guerrilla group's was returning to his rented apartment from a casino when he was shot at close range with a pistol, according to wire service reports quoting Frency police in Cannes.

He is believed to be the most senior Plo official to be short in an assassination attempt during several years of mysterious underground warfare between the PLO and the Israeli secret service and among rival Palestinian guerrilla factions.

The PLO headquarters in Beirut immediately blamed Israel for the attack. But diplomatic sources and other observers held out the possiblity that it could be connected with the seizure of the Egyptian Embassy in Ankara by Palestinian guerrillas earlier this month or with the internecine conflict among Palestinian factions which has erupted sporadically in waves of vengeful political assassinations.

Mohsen, a burly man with a taste for high living was a man with many enemies. He was a veteran of the shadowy world of political intrigue and violent power struggles within Palestinian guerrilla rangks, as well as a public figure engaged in PLO diplomacy.

He arrived in France Friday from Monrovia, Liberia, where he had headed the PLO's delegation to a summit conference of the Organization of African Unity. Despite that public appearance, Mohsen reportedly entered France secretly on a false passport, eluding detection by French police and intelligence services, which normally keep tabs on such persons.

Returning at about 1 a.m. from an evening at Cannes' Palm Beach casino, Mohsen was about to enter his fourth-floor luxury apartment on La Croisette Promendade, one of Europe's most expensive boardwalks, when a man hiding in a service stairway shot him in the head from a distance of about six feet, according to reports.

Mohsen's wife was opening the door for him and apparently witnessed the shooting, police said.

Two men, one described as Arab in appearance and the other apparently European, fled the building after the attack. They were being sought by French police.

In a statement issued in Beirut, the PLO high command warned, "This crime will not go unpunished."

Saying it holds "the Camp David alliance responsible for this crime," the Saiqa guerrilla faction vowed, "Our decisive reply will come sooner than expected."

The PLO office in Paris said the shooting "can only be the work of Israel or its agents" and reminded the French government of past assassinations of PLO officials "on French territory."

Mohsen's role as chief of the PLO's military department made him a potential target of the Isreli secret service, observers said, but he also hd acquired enemies in the Middle East and within Palestinian guerrilla ranks.

A prominent theory on the motive for the shooting, one apparently taken seriously by French police, is that it was done in retaliation for the recent Palestinian attack on the Egyptian Embassy in Ankara, Turkey.

Egyptian Prime Minister Mustafa Khalil vowed then that Egypt would strike back.

Commenting on the shooting of Mohsen, today, however, he said pointedly, "We always condemn terrorism in any form -- even against the enemy."

Despite the current animosity between Egypt and the PLO and Syrian leaderships, the possibility that the assassination attempt was the work of the Israeli secret service, Mossad, could not be ruled out, Middle Eastern observers said.

In January, Ali Hassan Salameh, alias Abu Hassan, 36, a flamboyant PLO guerrilla leader whose name headed Israel's most-wanted list, was blown up by a remote-control bomb as he was being driven from his Beirut home. The powerful blast also killed four Palestinian bodyguards and three Lebanese passersby.

Although the assassins were never identified, Mossad was widely believed responsible. The Israelis long have accused Abu Hassan of having plotted the 1972 Munich Olympies attack, in which 11 Israeli athletes were killed.

In the early 1970s, the secret war between Mossad and Palestinian guerrilla groups was blamed for a rash of assassinations in the Middle East and Europe, following the Munich Olympics.

In December 1972, the PLO's representative in France, Mahmoud Hamchari, was killed when a bomb connected to his telephone exploded in his Paris apartment. The next month Basil Kubaisi, a member of the Popular Front for the Liberation of Palestine, was gunned down on a Paris street by unknown assassins.

More recently, this band of underground warfare has flared between the PLO and more radical pro-Iraqi Palestinian factions.

In January 1978, the Plo Envoy to London, Said Hammami, was slain in his office. Other victims in the course of the year included the PLO representatives in Kuwait and Paris. Two gunmen who killed the Paris envoy admitted after their capture that they had been working for the late Abu Nidal, head of an Iraqi-based faction and an archenemy until his death last year, of PLO chief Yasser Arafat.

The conflict with Abu Nideal tended to unite the often quarrelsome factions within the PLO, papering over animosities that had sprung up during Syria's invasion of Lebanon -- supported by Saiga -- to aid the Lebanese Christian Militia against their Palestinian and Moslem leftist foes. CAPTION: Picture, Zuhair Mohsen . . . made many enemies