Libyan Oil Minister Abdessalam Zaggar said here early today that the measures announced by the Reagan administration yesterday were "an act of aggression" against his nation and that it would seek outside help to keep its oil fields running.

Reacting to President Reagan's call for American oil men to leave Libya, the Libyan official told reporters here that he intended to ask the Organization of Petroleum Exporting Countries for its assistance "against what we consider an illegal and unfair action."

Zaggar is here for a meeting of the organization's oil ministers, who are scheduled to consider the Libyan brief against the U.S. action later today.

Zaggar admitted that the withdrawal of the estimated 1,500 Americans -- oil men working for five American companies in Libya and their dependents -- would have an adverse effect on his country's oil production, which has fallen to less than half of its level of a year ago.

But he insisted Libya would still be able to operate its fields with replacements from other, unnamed, countries.

Observers here felt that despite what Zaggar said, Libya might experience considerable difficulty in finding replacements, particularly for the highly specialized experts American companies employ to provide services such as drilling and well maintenance for the American oil companies. In addition, Libyan oil fields are particularly difficult to operate because, unlike most fields in the Middle East, the oil has to be pumped out rather than brought out under its own pressure.

Last summer, a report circulated among Western European diplomats that the Libyan oil industry was 70 percent dependent on U.S. companies and experts, many of whom work for such non-American companies as the Italian Agip.

Zaggar assured the American oil men in Libya that they were free to leave if they wanted to. But he hastened to add that their safety was assured and that they would continue to enjoy good working conditions if they stayed on.

He repeated Libyan leader Muammar Qaddafi's denial that his country had sent a "hit squad" to the United States for the purpose of assassinating high-ranking American officials.

Americans interviewed in Tripoli, Libya, by The Associated Press generally doubted that Qaddafi would send assassination squads, mostly because of the consequences. The Americans also said they felt safe, earned more money than they could in the states and ignored anti-American propaganda.

Zaggar also said that Libya's relations with American oil companies had always been very businesslike and strictly commercial, adding he hoped that if they now decided to pull out of Libya under administration pressure they would do so legally.

Altogether, Zaggar struck a conciliatory note in his comments, which were made to journalists attending the regular year-end meeting of the oil exporters' association.

OPEC oil ministers failed last night to resolve disagreements on oil price differentials, price variations to take account of quality, but United Arab Emirates Oil Minister Mana Saeed Otaiba, OPEC's president, predicted they would reach agreement later today, Reuter reported.

It was not immediately clear what the reaction of the 13-nation group would be to Libya's appeal for help.

Oteiba told reporters at the end of the meeting last night that the organization "has nothing to do with politics and we don't want to involve OPEC in politics."

But observers felt that it was likely the organization would at least make some show of solidarity with Libya despite the misgivings of OPEC's moderate members about that country's politics and pricing policy.

These observers expressed doubt that OPEC was in a position to provide the kind of technical assistance Libya will need to replace the departing Americans. Their special expertise is not easy to find as they are in short supply and in great demand. More likely Libya will seek the necessary technical assistance from Soviet Bloc nations, possibly from both the Soviet Union and Romania.

"We will have help from many friendly countries," Zaggar said.

Earlier yesterday, Libya received the support of Iranian Oil Minister Mohammed Gharazi, who said in an interview that American companies that refuse to buy oil from the North African nation should be put on an OPEC blacklist.

Libya has been caught at a particularly vulnerable moment. Its oil production is already way below normal because of the glut on the world market and the high prices Libya has been asking for its top-quality crude.

The latest figures published by the American Petroleum Institute indicate Libyan production as of September had fallen to 700,000 barrels a day, down from an average of 1.8 million barrels last year.

Libyan exports to the United States have also plummeted from more than 400,000 barrels a day at the beginning of this year to only 274,000 barrels this fall -- about 5 percent of present total U.S. imports and only 2 percent of total American consumption.

The loss of even these exports, should the Reagan administration go ahead with an oil boycott against Libya, would be an added difficulty. However, oil analysts here said Libya could probably find alternative markets in Europe, provided it reduced its prices.

Libya is fighting here to keep the price of its oil at $37.50 a barrel. But Nigeria, which has a similar quality oil, is selling its crude at a full dollar less. For this reason Libya has been experiencing serious difficulties in finding customers for its oil.