Fighting to regain control over the world oil market, ministers of the Organization of Petroleum Exporting Countries today agreed to set their first formal production limits, establishing an overall ceiling of 18 million barrels a day while pledging to hold firm on prices.

In a separate action, Saudi Arabia, OPEC's dominant producer, announced an additional unilateral cut in its production of a half-million barrels per day, fixing its daily production for April at 7 million barrels. It was the Arab kingdom's third reduction in five months.

The combined result of today's moves at the conclusion of a two-day emergency conference represents a production drop to 17.5 million barrels a day. That level is about 1.5 million barrels less than what the cartel says it supplied during the first quarter of the year and slightly more than half the 32 million barrels OPEC pumped daily at the peak of its production just three years ago.

OPEC members also called on major oil exporters outside the organization, such as Britain, Norway and Mexico, to reduce their production rather than cut prices further. OPEC's president, United Arab Emirates Oil Minister Mana Said Oteiba, declared the organization's determination to protect prices, saying, "We are ready to go to 10 million barrels a day if necessary."

While holding the reference price at $34 a barrel, OPEC agreed to cut in half the differential it has allowed on higher grades of crude oil. These surcharges have allowed Libya and Algeria to set a price of $37 a barrel and Nigeria $36.50. Both the production ceiling and price decisions will be reviewed when OPEC holds its next regular meeting May 20 in Quito, Ecuador, officials said.

Industry analysts said today's decisions probably would have no immediate effect on gasoline and heating fuel prices, which have been falling for the past several months. But they said the actions could result in a slower drop.

Although the cutback, set to begin April 1, appears to be a substantial reduction of the available crude oil, industry experts here disputed both the real size of the decreases announced and whether the cuts would be enough to stop for long the drop in oil prices that has threatened in recent weeks to collapse OPEC's pricing system.

"I think they've put together a package which will work," said Ian Seymour, editor of the Middle East Economic Survey, an industry newsletter. Other knowledgeable observers, however, said that while the reductions might succeed in steadying prices in the short run, they would not prove sufficient to ease the glut through the entire year, particularly if world demand for oil continues to slacken.

"They haven't really seized control of the situation," said Dan Smith of London Oil Report, "though they might just luck through it."

The oil ministers heralded their decision as a demonstration that OPEC, facing what many regard as the cartel's most serious challenge in its 20-year history, can coordinate production and pricing among its 13 members to save itself from panic and disintegration.

Significantly, today's action marked the first time that organization members formally agreed to honor production quotas. Two previous attempts at drafting OPEC production schedules, in 1978 and 1981, were cast as informal gentlemen's agreements.

To help ensure that OPEC's members hold to their new commitment, the conference decided to establish a special committee, made up of ministers from the United Arab Emirates, Venezuela, Indonesia and Algeria, to monitor compliance. While no sanctions for violators were spelled out, the establishment of a self-policing group was seen as reinforcing the image of OPEC as a traditional cartel actively managing production.

Aside from the Saudi reduction, the most substantial cut in production will fall on Venezuela (down 355,000 barrels a day to 1.5 million) and on the United Arab Emirates (down 450,000 to 1 million). Three members--Iran, Iraq and Libya--were said to have been spared cutbacks.

"We came here conscious that the meeting would be difficult and it was difficult," said Venezuelan Oil Minister Humberto Calderon Berti, who stated that his country had finally agreed to substantial cuts in production in the interest of OPEC unity.

OPEC President Oteiba said, "I hope our conference here today is a turning point in the history of the industry and the end of the glut, and that the market will at this moment start to consolidate and be put on the right track."

The actual impact of the reduction is difficult to assess because there is disagreement within the oil industry over what OPEC's current levels of production are. Industry publications estimate output at 18 million barrels a day but OPEC ministers use a figure of 19 million.

Also in dispute is how much of the current oil surplus is due to weakened demand and how much is the result of market flooding by oil companies drawing down inventories at what OPEC officials claim is too fast a rate.

Company officials put the draw-down at less than 2 million barrels per day. But Saudi Oil Minister Ahmed Zaki Yamani, accusing unnamed forces of maneuvering to wreck OPEC's pricing structure, today insisted that the depletion of inventories was running at 4 million during the first quarter and was "well over" that figure in the past 11 days.

If inventories are depleted too fast, Yamani said, oil companies risk being caught short again as they were in 1978-79 when the Iranian revolution, and later, the Iranian-Iraqi war caused a sudden shortfall in available oil.

The Saudi cutback in production came separately because of the Arab kingdom's traditional refusal to discuss its production decisions within OPEC. But the Saudis, who account for about 37 percent of OPEC's production, had been under pressure from other members to go lower than the million-barrel-a-day cut it already had made this month.

The current run on oil prices was stimulated by a drop of $5.50 to $31 per barrel by Britain, a non-OPEC member, for its North Sea oil. In remarks to reporters today, several OPEC ministers appealed to Britain and other non-OPEC members, which together account for more than 60 percent of the non-Communist world's oil, to join in keeping production reduced and prices stable.