Oil prices on the spot market have taken what analysts describe as a "significant downturn" in recent days, signaling the likelihood of a slowdown in worldwide price increases.

Reports of the downward trend in spot market prices came as Iran increased its price by $2.50 a barrel yesterday and Venezuela announed a $2 rise effective next Wednesday. These followed similar increases by four Persian Gulf producers.

In the view of analysts, the spot market decrease is the first clear evidence that a softening of the worldwide market, long predicted for the first part of 1980, has arrived.

While the Saudi strategy of raising its prices in an effort to restore unity to the official oil price structure has not yet worked, there also are indications that the practical result of closing the gap between the spot and official prices will be to bring some stability to what has been a volatile world market.

The free "spot" market, where oil is traded outside the official price structure of the Organization of Petroleum Exporting Countries, dropped sharply this week after a slow decline over several weeks, market sources in Rotterdam and analysts here said.

The prices ranged between $30 and $33 a barrel, about $10 below the peak reached in November. The drop brought the spot market virtually into line with the official prices charged by major OPEC producers.

Among the reasons for the drop, analysts said, were a large worldwide supply of crude oil and a diminished demand resulting from a mild winter and recession, as well as the psychological effect of an announcement by Mexico last month that it plans a significant increase in its oil production.

An immediate effect of the drop, according to one U.S. source, should be to "stop suggesting to producers that their official prices are too low." In addition, he said, the development could signal a period of "relatively stable prices" and should return the spot market to its traditional function of balancing the world market.

Until mid-1979, the spot market was used by oil traders to make one-time purchases when they were caught short or to sell quickly, and sometimes at a loss, when they had a surplus.

When major disruptions and uncertainties developed in the oil market, largely because of a shutdown of production in Iran, the spot market took on greater importance and its prices soared to more than $10 above the official price per barrel. To take advantage of these prices, producers began diverting much of their oil to the spot market.

This pushed oil prices rapidly upward, playing a major role in the doubling of prices last year, and left producing nations without a unified pricing structure.

Along with this week's drop in prices, market sources said, acitivity on the spot market has also dropped back to its lowest point in months, with few large transactions.

The announcement by Saudi Arabia a week ago that it was raising its price by $2, to $26 a barrel, was seen by analysts as an effort by the world's biggest oil exporter to reunify the price structure.

At the December meeting of OPEC oil ministers in Caracas, at which producers failed to agree on an official price, the Saudis reportedly told other moderate oil countries that they would raise their prices.

Saudi Arabia's intention, observers here say, was to close the gap between its prices -- the lowest in OPEC -- and the $30-a-barrel prices of Libya, Algeria and other militant countries. The Saudis reportedly felt this would unilaterally bring unity and perhaps stability to OPEC's price structure.

Analysts are at a loss to explain the decision by four other Persian Gulf producers -- Iraq, Kuwait, Quatar and the United Arab Emirates -- to promptly raise their prices about $2 a barrel, putting them ahead of Saudi Arabia once more.

Yesterday, Iran announced a $2.50 a barrel increase, bringing its price to $31, a move that one analyst here called "clearly unwarranted" in light of other market developments.

The announced increases by the Persian Gulf countries have put pressure on North Sea producers, who have been asked by Britain to adhere to a $29.75 base price.

The drop in spot market prices is also expected to remove pressure on Britain to make major increases in the price of North Sea oil.

Calling for "moderation" in pricing, Britain has proposed a price of $29.75 a barrel. Major companies reportedly have agreed to this price, but some small independents urged prices comparable to those asked by North African producers.