President Reagan left the door open today to U.S. participation in global negotiations on world economic problems, a process sponsored by poor nations and Washington's allies, but set conditions that made it clear the United States remains a most reluctant partner.

In conciliatory words far different in tone from his speech on economic development in Philadelphia last week, the president said he takes seriously the commitment to move toward global negotiations. He underlined, however, that the United States remains opposed to a process that would give poorer nations more power in the world's economy.

Reagan's speech at the opening session of the 22-nation summit on international cooperation and development here was one U.S. officials want perceived as a step toward meeting the demands of other nations for global negotiations, but foreign leaders immediately fell to debating how large a step Reagan had taken.

Global negotiations have become the central issue of this summit and initial reaction ranged from pleasure that Reagan had not closed the door on further progress to suspicions that his stand was primarily a damage-limiting measure aimed to keep Cancun from ending with the United States standing alone.

U.S. officials said they anticipate a "very positive" reaction because Reagan is willing to continue the dialogue even if he does not support the global negotiations in a U.N. framework that the poor nations want. As a result of Reagan's speech, U.S. officials said, they expect that talk of a New International Economic Order, which the United States resists, will diminish.

Reagan referred to the carefully hedged words on global negotiations of last July's Ottawa summit of industrialized nations. He offered an explanation of how the United States understands the Ottawa phrase that a mutually acceptable process of global negotiation should begin "in circumstances offering the prospect of meaningful progress."

Reagan said that prospect should spring from four essential understandings: that the talks be aimed at specific development problems; that the existing financial institutions like the World Bank and International Monetary Fund not be altered and that their decisions not be made subject to any review; that talks focus on economic growth not reallocation of resources, and that the talks be held in a cooperative spirit.

Secretary of State Alexander M. Haig Jr. called the U.S. position "a constant evolution" and explained to reporters that Reagan's vaguely stated conditions are designed to rule out any new international structures such as an energy affiliate to the World Bank.

"We have enough international appendages today," he said.

Haig further explained Reagan's words to mean that the United States wants world economic growth, but not redistribution of wealth from rich to poor. Finally, he said, the United States will not take part in talks that are held in a divisive atmosphere. It appeared Haig was warning that if the United States were to become the target of criticism it would pull out.

However, the ambiguity of the U.S. effort to appear to take a step forward while not making any major commitment was underlined when White House chief of staff James A. Baker III told reporters that "the position is exactly the same as it was in Ottawa." Although other nations were praising the United States tonight for advancing from its Ottawa position, Baker insisted there had been no movement.

One important obstacle to global negotiations is that no one is certain what they are. West German Foreign Minister Hans-Dietrich Genscher, for example, today told the summit that Cancun should "signal a green light for the opening of global negotiations" while a West German official said one important activity at Cancun should be defining what global negotiations are.

British Prime Minister Margaret Thatcher and French President Francois Mitterrand addressed the need for a better definition of the negotiations.

"The term seems to mean different things to different people," Thatcher said, in a statement which was less definite in its support of the negotiations than her foreign minister had been yesterday.

Mitterrand appealed for study of what global negotiations means, including what role the United Nations and existing international organizations should have.

If Cancun is seen as the conference that gives a green light, it will have answered the hopes of many leaders of the developing world.

Brazil's Foreign Minister Ramiro Saraiva Guerreiro ended his opening speech today with a call to break "the impasse which has up to now prevented the launching of global negotiations."

Although the United States had company from its European allies at Ottawa in its wariness of global negotiations -- where there would be pressure for decision-making on the basis of one-nation, one-vote -- the NATO nations, including Great Britain, all now support the process, increasing the U.S. isolation.

U.S. officials made it clear that Britain's switch was the unkindest cut. A Cabinet member, informed of British Foreign Secretary Lord Carrington's unequivocal support for global negotiations and for a new international energy agency, said yesterday: "Thanks a lot. That's just what we need."

Treasury Secretary Donald T. Regan said the United States had made only one initiative at the afternoon session, which was entirely devoted to food and agriculture. President Reagan offered to send volunteer teams of U.S. farmers and agricultural experts to developing countries to train farmers. This would not cost the U.S. government anything under present planning and was greeted with warm approval at the summit, Regan said.

The summit will continue Friday with one hour of discussion on trade, one hour on energy and an hour on financial problems. The final session the afternoon has no designated topic.

One specific, but not new, offer came from Japan's Prime Minister Zenko Suzuki. He pledged that Japan will attempt to double the $10.5 billion foreign aid it provided in the last five years during the next five years.

Television cameras were permitted to film the opening speeches by the summit's cochairmen Lopez Portillo and Canadian Prime Minister Pierre Trudeau.

After Trudeau spoke, the summit continued in private with the morning session lasting more than an hour longer than scheduled.

Reagan's proposals, although couched in language leaving plenty of room for future interpretation, were closely studied since the president had prepared for Cancun by making two speeches that turned a cold shoulder on global negotiations in favor of defending the U.S. record of aiding the world's poor and urging that developing nations seek progress through private investment and free trade.

Once again here, Reagan praised private economic development, saying, "History demonstrates that time and time again, in place after place, economic growth and human progress make their greatest strides in countries that encourage economic freedom."

"I am puzzled by suspicions that the United States might ignore the developing world," Reagan said. "We have provided $57 billion to the developing countries in the last decade." In the last year, he added, "we extended almost twice as much official development assistance as any other nation."

Critics of the U.S. effort measure aid in percentages of gross national product not in absolute sums as Reagan does. U.S. concessional aid is now 0.27 percent of GNP. In addition, a very high percentage of U.S. aid goes to a small number of countries, particularly Egypt and Israel.

"At a time when we have to cut back food stamps at home because of a tight budget, the idea of 1 percent for foreign aid is totally unrealistic," a high level official said. Some poor nations have spoken of setting 1 percent of GNP as a target level for aid from the wealthy nations.

On trade, Treasury Secretary Regan introduced a sign of new flexibility by saying the United States will be prepared "as time goes by" to discuss a liberalization of the "multifiber agreement" which restricts textile imports.

Nigerian President Shehu Shagari took exception to a Reagan story illustrating the advantage of training over aid.

If you give a man a fish, he is hungry the next day. If you teach him how to fish, he will never be hungry again, Reagan said in his Philadelphia speech.

"There is an object lesson in that story for all of us," Shagari said. "I must, however, hasten to add that that man has to be supplied with hooks and net in order to put into practice his invaluable knowledge of fishing."

Shagari outlined Africa's mounting food crisis to the summit. By the year 2000, Africa's food import bill will be $8.6 billion for cereals alone, he said. "Africa's food bill presently equals its bill for oil," he added.

Reagan's warmest words were a pledge of friendship, but he did not share Lopez Portillo's sense of urgency.

"Together we can identify the roadblocks to development, and decide the best ways to stimulate greater economic growth everywhere we can." Reagan said. "The dialogue will go on. The bonds of our common resolve will not disappear with our jet trails" when the summit ends.