Former Syrian premier Salah Eddin Bitar, the most prominent Syrian opposition leader in Western Europe, was assassinated here today by a gunman who shot him in the back of the neck with a silencer-equipped pistol as he was unlocking his office near the Arch of Triumph.

The killing followed by just three days an attempt here by a five-man squad to assassinate former Iranian premier Shahpour Bakhtiar. Police sources said that the squad's commander claimed that he had been personally assigned the mission by Palestinian leader Yasser Arafat on behalf of "the Iranian authorities."

Several veteran Western observers ofthe Palestine Liberation Organization scoffed at the idea as a bizarre attempt to throw investigators off the track. The PLO vigorously denied the story and French official sources indicated they accepted the denial.

Observers took seriously, however, the suggestion that the hard-pressed regime of Syrian President Hafez Assad, despite its ambassador's denials here, was behind the slaying of Bitar, a man who had come to be the leading symbol of democratic opposition to an increasingly repressive government.

Bitar, 68, was by far the best-known leader of a group of exiled opposition forces that had begun about six months ago to overcome their differences and to organize themselves here into a broad multireligious National Front against the Assad government, which is based on the small Alawaite Moslem sect. The assassination of Bitar, said a qualified Western diplomatic observer, is "a sign of the desperation of the regime."

The government-directed press in Damascus has recently stepped up its threats against opponents of Assad.

Two weeks ago Syrian radio said that "Syria is capable of firmly striking at all the hands attempting to harm it" and added that the Syrian people "believe it is high time to launch a comprehensive attack" on opponents of the government. A few days later the official Al Baath newspaper warned, "The hand that is directed against Syria will be cut off be it directed from abroad or internally."

This evening, no one had claimed responsibility for the killing of Bitar, however. The Syrian ambassador in Paris expressed his sorrow and, denying any official complicity, said such a killing would do far more harm to Syria than Bitar's writings.

Witnesses said they saw one man rush from the building soon after Bitar was shot shortly before noon. Bitar was killed by the single bullet by the assassin who apparently waited for him in the hallway of his office on the Avenue Hocne.

French-educated, Bitar had been largely in exile since 1966. A Sunni Moslem, he was the cofounder with the Christian Michel Aflag of the Baath Socialist Party, who later fled to exile in neighboring Iraq, with its rival branch of the Baath, while Bitar went to Lebanon, and later the West.

In ais, Bitar ran an "intellectual" journal called Arab Renewal.

Another leader of the anti-Assad group is Hammoud Choufi, formerly Syria's ambassador to the United Nations, who defected last December and said he would help lead efforts to topple Assad.

[Choufi said in New York Monday that he had learned that he, Bitar and Issam Attar, a leader of the opposition Moslem Brotherhood living in exile in West Germany, were on a special "hit list" drawn up by Assad.]

[Choufi accused Assad of "blatantly resorting to the savage tactics of murder and assassination of his political opponents" in a "desperate attempt to quell the mounting resistance he is facing from broad sectors of the Syrian people."]

Bitar was Syria's foreign minister from 1956 to 1958, a leading minister in the short-lived United Arab Republic of Egypt and Syria headed by Gamal Abdel Nasser, and premier of Syria during most of the period from 1963 until his exile three years later. While in power, he represented the relatively liberal faction of the Baath Party opposed both to the Nasserites from whom they seized power and to the Baath military hard-liners who succeeded the Bitar group in a coup.

Bitar was condemned to death in absentia, but pardoned by Assad 10 years ago. Bitar returned home briefly in 1977.

His name became a rallying point for middle-class and professional opposition demanding free elections and the restoration of democratic rights from a government prey to opposition from a variety of quarters.

Assad himself has survived at least two assassination attempts in a little more than a year, one apparently organized from Iraq and the other by the archconservative Moslem Brotherhood.

Bitar's reemergence as an important political symbol apparently worried the Assad government. It is reliably said to have sent a high-level emissary to France recently to ask that he be expelled.

France has had especially close relations with Syria. Assad's brother Rifat, commander of the division that is the government's special guard in Damascus, has spent much time in Bordeaux in recent months.

The French police unions renewed their calls for reinforced protection for political exiles and for their police guards after the Bitar assassination. They had reacted strongly after the foiled attempt on Bakhtiar on Friday in which one policeman was killed and another critically wounded.

In response to police union pressure, Interior Minister Christian Bonnet pledged that the Bakhtiar attackers would be put on trial here and not extradited. They were indicted for murder yesterday.

Anis Naccache, 29, the leader of the squad, reportedly told police that he had accepted the assignment to kill Bakhtiar "as a soldier of Fatah" -- the main-line PLO organization headed by Arafat.

The apparent insistence from the start by the five prisoners that they are members of Fatah is seen by experienced observers here as an attempt both to draw attention from other, more extremist groups who may have been involved and to embarrass Aarafat, who enjoys good relations with the French government.