Secretary of State Cyrus Vance, in a major and forceful statement of U.S. policy toward Latin America, said today that U.S. economic aid and trade would be linked to Latin nations' observance of human rights.

U.S. "cooperation in economic development must not be mocked by consistent patterns of gross violations of human rights," he told the opening session of the annual conference of the Organization of American States.

Speaking to a closed session of he OAS, Vance stressed that "there is no ambiguity" in OAS members' commitments to observe those rights. He called on each of the 25 member nations to grant free access for investigations of violations to the Inter-American Commission on Human Rights and for the first time offered such access in the United States.

The statement was the most uncompromising and harshest the Carter administration has yet delivered to Latin America on human rights.

Previous administration speeches, including Carter's April OAS Latin policy speech and comments ny U.S. Ambassador to the United Nations Andrew Young, have voiced U.S. disapproval of actions of some of Latin America's rightist military governments, but the criticisms have always been overshadowed by U.S. stress on positive programs of cooperations. Vance's speech contained none of this positive outlook.

In equally stong terms, Vance also called for a "thoroughly reformed OAS structure," including cutting the organization's bureaucracy, merging its three councils into one, and replacing much of its standing infrastructure with informal conversations.

He reiterated an earlier U.S. recommendation to cut Washington's share of the OAS budget to only 49 per cent, down from a current 66 per cent, which he described as An an-anchronism.

The comments were Vance's opening remarks to the week-long conference of foreign ministers and OAS delegations on this tiny Caribbean island, the smallest and poorest OAS member.

Unlike previous years, this year the main issues on the OAS calendar are not economic ones that cast the United States as a hemispheric villain, but precisely those that Vance outlined - human rights and a reorganizaion of an organization that some U.S. officials have called "useless."

In a passage that several delegates called special attention to, Vance noted that "since the last general assembly" in Santigo, Chile, a year ago, "men who once sat among us have been victims of violent assault. We mourn the deaths of [El Salvador] Foreign Minister [Mauricio] Borgonovo, and former [Chilean] Foreign Minister Orlando Letelier."

Borgonovo was kidnaped and murdered by leftist terrorists last month. Letelier was killed by abomb explosion in Washington last September. Chile's ruling military junta has been widely accused of the crime, for which there have been no arrests.

During a two-day visit to the conference, Vance is to meet individually with at least 20 Latin foreign ministers, driving home Carter's message.

"The main purpose of the administration now," said one high-level U.S. official, "is to speak with one voice." Admitting that apparent policy flip-flops have caused "some doubts about the U.S. government's seriousness of purpose," the official said Vance will be trying to clarify what the administration now believes is a substantive hemispheric policy.

While the smaller OAS nations are expected to appeal to Vance for new hemispheric concessions in aid and trade, the more powerful and wealthy countries are primarily interested in bilateral problems.

In one resolution before the assembly, Venezuela and Ecuador have demanded admission to the 1974 generalized system of U.S. trade preferences, from which they are excluded under a provision barring oil-exporting nations.

Argentina denied some U.S. military assistance becuse of human-rights violations, clearly plans to confront Vance on the rights issue. Argentina has consistently maintained that its internal war against terrorist guerillas necessitates harsh measures, and it has rejected international accusations of illegal imprisonment and torture as "interference."

Today Vance said that "If terrorism and violence in the name of defense cannot be condoned, neither can violence that is officially sanctioned. Such action perverts the legal system that alone assures the survival of our tradition."

His statement echoed that of Treasury Secretary Michael Blumenthal, who earlier this month told delegates to the Inter-American Development Bank conference that the United States does not believe terrorism can be used to combat terrorism.

Vance said the United States will vote to increase the budget of the OAS human-rights commission.

During this assembly, the commission is to present reports on Chile and Cuba. Last month Uruguay said it would not "invite" the commission for an on-site inspection of its prisons, despite heavy OAS pressure to do so. Paraguay has not yeat responded to a similar commission request.

Vance's call for OAS reorganization was not a new one; it was unofficially proposed by Henry Kissinger at last year's meeting. Although the OAS has rarely suffered a lack of major issues, the United States believes that it has been less thing successful in resolving those instige.

There is little purpose to the hundreds of council and committee meetings held each year, mostly in Washinton, a U.S. official said, because "Most of the Latin foreign ministers don't follow what goes on in the OAS. I can understand that, because so little does."

It was avoidance of controversy that determined the location of this year's general assembly meeting.

Realizing that Grenada had neither the facilities nor security for such a meeting, the majority of members privately suggested it be held in Washington.

Argentina, among several others, privately objected because, according to sources, it did no relish the prospect of confronting the U.S. and international Washington - based press corps.

Thus, the meeting is being held in the one-story, beachside Holiday Inn hotel here, with Grenada's colorful Prime Minister Eric Gairy acting as host. Gairy opened today's proceedings with an appeal to the OAS memhership to trust in God, and asked for special OAS attention to the activities of extraterrestial beings and flying saucers.

To nearly everyone's amazement, Grenada's 120,000 people have risen to the task of handling more than 2,000 visitors of one time, thanks to a portable geodesic-dome convention hall brought from Cape Canaveral and gifts of 700 chairs from Canada, cars from Japan, stationery from Chile, souvenir briefcases from Venezuela and secretaries from neighboring Barbados.

There were still complaints from some delegates, however, most of whom were required to double or triple up in Grenada's limited hotel rooms. Yesterday early conferance arivals had to hold their informal meetings without having had morning showers. For most of the day, there was no running water in Grenada.