President Reagan's speech yesterday to the Organization of American States committed his administration and his personal prestige more clearly than ever before to the complex and controversial policy of anti-leftist activism in Central America.

Beyond the details of the long-promised Caribbean basin economic support program, much of which had become known during the eight months of its bureaucratic incubation, Reagan's address was notable for his very tough rhetoric in describing the security problems of the area and his avoidance of details or even strong clues about a future U.S. military response.

To justify the increased aid and economic concessions as well as the increased U.S. military support and involvement, Reagan described the U.S. "vital interest" in Central America as an area close at hand and an artery for imported oil and other trade. It was unclear from the immediate reaction whether this appeal will override political resistance from several quarters on Capitol Hill.

"Let our friends and our adversaries understand that we will do whatever is prudent and necessary to ensure the peace and security of the Caribbean area," declared Reagan in addressing future policy. He did not go beyond this very general statement, despite expectations raised in last Thursday's presidential press conference when he turned aside several pointed questions because of the forthcoming speech.

According to a senior official who was involved in the preparation of the speech, the words "prudent and necessary" were chosen to convey that U.S. engagement will be "not open-ended but very firm."

Secretary of State Alexander M. Haig Jr., in much-quoted testimony before the Senate Foreign Relations Committee on Feb. 2, said the United States would do "whatever is necessary" to contain the guerrilla threat to El Salvador. In a Dec. 4 policy statement to the OAS general assembly in St. Lucia, Haig said the United States would join others to do "whatever is prudent and necessary to prevent any country in Central America from becoming the platform of terror and war in the region."

In keeping with a White House decision early last year that personal involvement with the El Salvador problem could be politically damaging to the president, Reagan until yesterday had had little to say in public about the growing U.S. involvement in Central America. Haig, presidential counselor Edwin Meese III, Secretary of Defense Caspar W. Weinberger and lesser officials spoke out while Reagan stood aloof.

The benchmark of presidential involvement in highly visible fashion came at a time of increasing domestic political controversy about the El Salvador-Nicaragua policy and a moment of great uncertainty about what happens next in the region. As the situation is described by officials, a great deal depends on whether intensified guerrilla operations in El Salvador disrupt the March 28 national elections, and on the relative fortunes of rightist and centrist elements in that balloting.

Reagan's charges against Nicaragua, which he described as "a platform for covert military action" for almost two years, appear to set the stage for heightened tension with that country. Reagan did not address reports that he has authorized support and encouragement for undercover paramilitary operations against Nicaragua.

In a briefing for reporters on the Reagan speech, Assistant Secretary of State Thomas O. Enders charged that in the last six weeks the flow of arms from Nicaragua to El Salvador has "increased enormously" to levels that are "by far the highest that have been known," exceeding even those reported before the guerrilla offensive of January, 1981.

Enders' statement went beyond previous U.S. estimates of recent weapons flow, and was said by officials to be based on new intelligence. In the face of continuing denials by the Nicaraguan government and skeptical questions from reporters, however, neither Enders nor any other official would produce the evidence to back up their charges.

Following a series of White House briefings in advance of the speech, some members of Congress strongly endorsed the Caribbean basin economic initiatives, others were generally favorable but less enthusiastic, and some predicted that the plan will run into trouble due to resistance to foreign aid at a time of heavy domestic budget cuts.

Some Farm Belt legislators also expressed concern about the duty-free imports of Caribbean foodstuffs envisaged in the plan.

Sen. Charles Percy (R-Ill.), chairman of the Senate Foreign Relations Committee, predicted the plan will receive "strong bipartisan support" and said he will schedule hearings on it as early as mid-March.

House Speaker Thomas P. (Tip) O'Neill Jr. (D-Mass.) said he believes Reagan is "on the right road" and that the program is likely to pass the House despite opposition based on U.S. economic conditions.

Senate Democratic Whip Alan Cranston (Calif.) applauded the principles of plan but opposed expanded aid to the Salvadoran junta while it refuses to negotiate with the insurgents.

Sen. Larry Pressler (R-S.D.) said, "It's going to be very tough to come up with the money, to put it bluntly."

There seemed little doubt that Reagan's personal involvement on Capitol Hill, matching his new visibility on the trouble in Central America, will be required to translate yesterday's speech into law.