Prime Minister Indira Gandhi opened a meeting of Third World countries here today with an attack on the industrialized nations, especially the United States, for failing to continue the global negotiations on international economic problems begun at the summit in Cancun, Mexico, last October.

In her opening speech this morning, Gandhi said, "The visible deterioration in the global economy" since the Cancun session "can have disastrous results for humankind" if it is not checked. Although the 22 leaders of industrialized and developing countries at Cancun agreed on the need for a quick start to global negotiations, she said, "subsequent developments are disappointing."

She blamed the industrialized nations for deepening the plight of the less developed countries with their efforts to climb out of the global slump.

Without mentioning the United States by name, she pointed in two speeches to Reagan administration policies permitting interest rates to soar, reducing foreign aid and bringing about cuts in low-interest loans to poor nations as being especially harmful to the underdeveloped countries of the Third World.

"They are the ones who suffer the most from the decisions of the few who dominate the world," Gandhi told a group of 44 Third World nations gathered here in what has come to be known as "a South-South dialogue."

Gandhi called the meeting an effort to break the deadlock in discussions between the industrialized nations of the "North" and the less developed countries of the "South" over restructuring the world's economic order to help the have-nots.

The Cancun agreement stated that all 22 participants favored global negotiations within the United Nations on world economic problems, but gave little detail on how or when those negotiations would begin.

The Reagan administration, for instance, has offered only limited acceptance of the concept.

Instead of trying to improve their economic positions on their own, Gandhi said the industrialized world should help build up the developing countries.

"Progress in the developing countries," she said, "will help to rejuvenate the stagnating economies of industrialized countries. In turn, improvement in the economies of the industrialized world offers better prospects for our own development."

While the meeting was originally envisioned as being on the policy-making level, most of the countries were not represented by their leaders and instead sent lesser ranking officials. One key nation, Saudi Arabia, declined to attend because it said the timing of the conference was inconvenient.

The conference was set to coincide with the awarding of the second annual Third World prize to Tanzanian President Julius Nyerere, who said that the "unjust and exploitive international economic system is in the process of falling apart . . . . The law of the jungle is returning."