Amid tumultuous, well-choreographed demonstrations in the heart of the city, Syrian President Hafez Assad arrived here today to put his own personal seal of approval on his country's "unconditional" union with Col. Muammar Qaddafi's Libya.

Flying in from Damascus with a high-level government delegation in response to Qaddafi's call last week for unity between their two countries, Assad delivered a wholehearted endorsement of the merger after receiving a warm embrace from a jubilant Qaddafi.

The exact nature of the union -- a full economic and political merger or simply a loose federation -- has yet to be made clear.

Nonetheless, Assad's embrace of Qaddafi appeared to dash hopes, once high in Washington, that he could eventually be brought into the peace negotiations process. Western diplomats here said his alliance with Libya, the most uncompromising of the so-called Steadfastness Front Arab states, seemed to commit Syria to an equally belligerent course in the future. s

While underlining that the merger of the two nations responded to the deep imperatives of a pan-Arabism felt "by all Arabs," both Qaddafi and Assad made it clear their union was also a direct answer to the threats they feel to their security resulting from rapprochement among Israel, Egypt and the United States in the wake of the Camp David accords.

One immediate result of the union may be as much as $3 billion in Libyan financial assistance to the economicically hard-pressed and increasingly isolated Assad government.

"On behalf of the Syrian people, on behalf of your families in Arab Syria, I say to you we are with you with no hesitation on the road of unity and liberation," Assad told a cheering Libyan crowd gathered to greet him.

"For the sake of confrontation we seek unity, for the sake of steadfastness we seek unity, "Assad continued. "For liberation we seek unity, for Palestine we seek unity, for Arab dignity we seek unity, for freedom we seek unity, for Arab socialism we seek unity -- and for the sake of unity we seek unity."

Neither Qaddafi nor Assad, who drove into town together in an open limousine to preside over what was probably Tripoli's biggest mass demonstration, shed much light on the exact nature of the union they have endorsed.

But both made it clear that they were responding to the threats they feel the Arab world faces as a result of the Camp David accords. They soundly denounced Egyptian President Anwar Sadat, Israeli Prime Minister Menachem Begin and the United States as the "enemies" of the Arab world.

The precise definition of the union between their markedly different nations, Libyan and Syrian officials insisted, were "secondary matters" which would be worked out later in negotiations. These officials said the important thing was that both Assad and Qaddafi had agreed, with no conditions, on the principle of total union.

Qaddafi, however, minced no words on the importance he saw in the union. As he introduced Assad to a shouting and cheering crowd of several hundred thousand Libyans -- mobilized from around the country for the occasion -- Qaddafi called the union a "testimony of Arab determination for life in preparation for death."

"To this unity we want to declare the entire Arab nation's mobilization," Qaddafi said as Assad stood by his side in the second-floor window of the former revolutionary command headquarters here. "We have to raise the call for sacrifice, martyrdom and death. We want to seek liberation."

Qaddafi specifically singled out the "U.S. bases and bridges" that had been established in the heart of the Arab world -- in Egypt, Somalia and Oman -- and called them a threat and humiliation to the entire Arab nation.

The readiness with which Syria, an economically deprived nation of eight million, has shown itself prepared to merge with Libya, an oil rich nation of about 3 million people, is an indication above all of how isolated Syria has become in the wake of Camp David.

Syria has lost the support of Egypt, its former chief ally in the confrontation with Israel. More recently, it also has broken with neighboring Iraq and has seen that traditional rival join in a previously unimaginable alliance with Saudi Arabia.

Beset by crushing economic problems mired down in a dangerous peace-keeping effort in Lebanon and threatened by serious internal dissent, Assad clearly sees in unity with Libya a means of breaking his isolation in the Arab world, as well as assuring a vital economic transfusion from Libya's oil wealth, running now at about $16 billion a year.

Qaddafi indeed, is reported to have insisted on unity, which meets his own personal messianic pan-Arabism, as a condition for boosting the Syrian economy.

Last April, in a meeting here among the so-called Steadfastness Front nations of Libya, Algeria, Syria, South Yemen and the Palestine Liberation Organization, a decision was made to provide Syria with more than $6 billion in economic support, half of it pledged by Libya. However, Libya has contributed only a fraction of that so far and has insisted on union as a condition for providing the rest, according to sources here.

Given the failure of past efforts at Arab unity -- between Egypt and Syria in 1958, between Egypt and Libya in 1972 and Libya and Tunisia in 1974 -- there is a good deal of skepticism about the future of the present merger.

Libyan officials, however, insist that Libya has learned much from unity failures in the past. It intends to build on the positive aspects of those failed unions, they say, and will not insist as in the past on immediate integration of the two state's difference political and economic systems.

"Unity now is an overriding necessity that will require a lot of work and sacrifices from us all," said Saab Mujber, a senior official of the Libyan Foreign Secretariat. "The main thing is that union take place."

"Unity is like a big castle in which we must all gather for safety," Mujber said in an interview. "If we can't get in one gate, we must go around to another one, and if that doesn't work then we must find a way to climb over the walls. There is no question that we have to get inside -- and find unity -- if we don't we die."