The prince who's putting Saudis center stage. Experts eager to see what support he gets for Fahd's peace plan

When he walks into a room, his tall, regal presence permeates the room, his handwoven mohair bisht robe and white cotton gutra flowing as he strides in to take command.

Saud al-Faisal appears very much the prince. But in a family that has, literally, hundreds of princes, rank alone is not enough to give him the status he now holds in Saudi Arabia.

It is instead Prince Saud's emergence as a major force in international diplomacy that has made him the man to watch in the Arab world, and a symbol of the future for this desert kingdom.

The Saudi government is now pinning much of its hope of rallying support for its new eight-point Middle East peace plan on its young foreign minister.

The proposals justifiably carry the name of Crown Prince Fahd, the chief administrator and idea man in the oil-rich nation. But it is Prince Fahd's savvy and sophisticated nephew, Prince Saud, who is being watched by diplomats and interested parties to see if he can convince the Arab world and the West to support an initiative to replace what the Saudis feel is the limited, and exhausted, process of Camp David.

His impact was first noticed by outsiders in January last year during a visit by Lord Carrington to this dusty-toned capital. After a quiet conversation with the soft-spoken Prince Saud, 22 years his junior, the British foreign secretary was ''converted'' into an active proponent of involving the Palestine Liberation Organization (PLO) in the Middle East peace process, according to members of Carrington's entourage.

Five months later, Lord Carrington was one of the main lobbyists behind the European initiative on the Mideast known as the Venice Declaration, which called for PLO participation. And last week, during another visit that included four hours of talks with Prince Saud, he in effect endorsed the Saudi plan on behalf of the 10-member European Community as a good basis for new negotiations to solve the Arab-Israeli conflict.

The next step for the Fahd plan is a vote at the 21-nation summit in Fez, Morocco, later this month, potentially a very significant meeting.

The government is said to be ''extremely bullish'' about the results of what all sides believe will be a heated conference: Prince Saud, the chief link man and lobbyist, is one of the reasons.

''He will be campaigning like mad,'' said one well-connected diplomat here, ''laying the groundwork to convince those states that are marginal, trying to swing others around. Fahd will not step in until positions have crystallized.''

The Saud-Fahd combination is, indeed, evolving into the most powerful in the 39 years that this collection of former fiefdoms has been a united nation.

The crown prince is a man of well-timed persuasion in the old-style convoluted way of Arab politics that one resident observer compared to 18 th-century France, while Prince Saud is a man of smooth logic and pragmatism, less an advocate of the checkbook diplomacy of his uncles in situations like Fez.

Specifically, Saud's assignment is said to be determining if any bloc of states favors amending the proposals, which basically represent a composite of UN resolutions, plus a guarantee that all states in the region shall have the right to live in peace - tantamount to Arab recognition of the Jewish state.

It is the latter that is the most attractive aspect, indeed viewed as a breakthrough, in the eyes of the West. But it is also the most controversial point in the current posturing of leftist Arab states for whom recognition of Israel is as much anathema as talks with the PLO is to Israel.

In his six years as foreign minister, Prince Saud has built up a host of invaluable contacts in the Arab world as emissary for his frail uncle, King Khalid. And personal rapport is the foundation of inscrutable Arab politicking.

For example, he is expected to be influential in countries like Algeria, one of the members of the so-called ''steadfastness front,'' the most militant alliance in the Arab world, because of his previous job as special envoy at talks on the Algerian-Moroccan dispute over the Western Sahara.

Prince Saud's role as the legman in the peace initiative has already been evident. Early last month he was dispatched by the King and crown prince to New York to appeal to the UN General Assembly to adopt a resolution based on the Fahd plan.

Again he was laying the groundwork. Pending Arab approval, the Saudis hope the forum for negotiations based on the Saudi proposals rather than a Geneva-style conference.

But it is perhaps in the long term that his dynamic and outgoing style are more important. It is one of the crucial strengths of the third son of King Faisal - who bears an uncanny, though more dashing, resemblance to his late father, right down to his long spindly fingers - that he scores well in dealings with both Arabs and Western governments.

The major events on his calendar this month might impress even the once ever-shuttling Henry Kissinger. Over the weekend he was off to Beirut in an attempt to revive the Arab followup committee that is seeking to end six years of political turmoil in Lebanon, then returned for the six-nation Gulf Cooperation Council in Riyadh this week. Somewhere along the way, US sources said, he will probably meet with US special envoy Philip C. Habib, who is returning to the region this month. All that, and Fez, too.

Prince Saud is the first - and so far only - grandson of the Saudi founder and patriarch Abdul Aziz Ibn Saud to come to the visible forefront of power, despite the dominance of his 42 paternal uncles.

And he is widely credited with pushing the traditionally insular and reluctant royal family to use its potential clout in the Middle East, notably in helping orchestrate a cease-fire between the PLO and Israelis last July. In the past, the kingdom has been unwilling to tie itself to anything more controversial than oil prices.

His early foreign exposure, including a BA in economics from Princeton and participation in UN delegations, is said to be partially responsible for his success. ''He can sustain a witty, 'Western' conversation, which makes him appear a less remote and haughty figure than some of the older generation of princes,'' a Western official here said.

Saud's style and shrewdness are also cited by some foreign diplomats as a reason that the kingdom will not follow - as has often been predicted lately - Iran and Egypt as a victim of US political largesse and military dependence.

Indeed he is considered to be realistic and often skeptical about Western ties. Before a meeting with US Secretary of State Alexander M. Haig, Jr., he was quoted as saying: ''If the role of the superpowers toward our cause is constructive, there will be no obstacle to good relations with them.

''But we deceive ourselves if we ever consider that our relationship with the great powers was based on friendship.''