At first glance, the marathon meeting of the Organization of Petroleum Exporting Countries now under way in London has the trappings of past OPEC sessions. The setting is a luxury hotel, heavily guarded by police. Into the evening, limousines deposit and collect delegates who toss off comments--often contradictory--on the deliberations as they sweep by hordes of reporters and cameramen.

But this is no routine gathering. These are OPEC's darkest hours in a decade. The group is thought to be in danger of falling apart, a casualty of the changing global economy and its own factional disputes. Even if the 13 members states do finally forge an accord, oil experts believe there is little likelihood it will stick.

"OPEC has no clothes," said the London Times today.

Ministers began to arrive in London a week ago, many expressing optimism that a deal was in the offing. Having failed in recent formal meetings to find an acceptable formula for adjusting prices and output to sharply reduced world demand, the envoys portrayed their discussions here as informal, in hopes of assuring maximum flexibility. Saudi Arabian Oil Minister Sheik Ahmed Zaki Yamani said an agreement was "imminent."

Instead, the talks have dragged on apparently without significant progress. The delegates have become visibly weary and uncertain. Each report of agreement has soon been overtaken by new signs of trouble. Iran has been adamant in public against any price cut from the present base of $34. Nigeria, which unilaterally set its price at $30, has resisted attempts to nudge it upward. There are serious squabbles over apportioning quotas.

Moreover, the realization is sinking in that the differences among members are so fundamental and OPEC's economic clout so diminished that the real price of oil eventually will be set in the marketplace no matter what happens here. Important oil producers outside the organization--the Soviet Union, Britain, Norway, and the United States--have dropped their prices in recent weeks (the Soviets twice) and more than half of OPEC's members are selling at cut rates, with sales still falling.

OPEC's best hope for a viable accord, specialists believe, would be to fix a price of $27 or $28 and a quota of 15.5 million barrels a day--figures well below what was being suggested when the London session began. While OPEC members have occasionally disagreed among themselves in past years, offering oil at several prices, to do so now would encourage further cuts from nonmembers and deepen the organization's factional disputes.

Any final arrangement that is plainly a jerry-built compromise with exceptions, loopholes and subclauses may preserve the form of OPEC, but not the substance as members quickly begin scrambling for advantage.

Does all this mean that OPEC is finished? Perhaps not. Even so unsatisfactory an outcome as the current meetings are expected to produce is likely to be regarded by delegates as better than no accord at all on the grounds that events in the months ahead--a significant upturn in the international economy, for instance--might again increase demand and give OPEC a new lease.

To leave London without any semblance of accord, ministers led by Venezuela's Humberto Calderon-Berti are arguing, guarantees that whenever another attempt at coordination is made, the base price for oil will be even lower than is being contemplated now--possibly $25 or less.

OPEC flourished at a time when oil producers decided to reap benefits from natural assets that they felt were being exploited by consumer nations and the big oil multinationals. The relationship of its Middle Eastern, African and Latin Americans members was always a function of mutual economic interests. While these have eroded recently, they have not disappeared altogether.

That is the reason, oil analysts say, why the countries--including warring Iran and Iraq--have endured this prolonged bargaining and may well try again if their present efforts founder. Alone or in small groups, the countries, even the once predominant Saudis, have substantially less influence over the oil trade than they have together.

So while competition and factionalism have increased in proportion to the developing oil glut, the reasoning that drew the OPEC countries together in the first place is still valid. Even though the London meetings are clearly the most difficult in OPEC's history, they are almost certainly not going to the final ones.