President Reagan will go before the Organization of American States today to present his long-awaited Caribbean basin development plan aimed at combating communist influence in Central America and the Caribbean through increased aid, trade preferences and investment incentives.

White House communications director David R. Gergen, while acknowledging that the speech will deal with regional politics and security, said "the emphasis is on what we can do to improve economic conditions" in that area.

Following a briefing on the plan for Republican congressional leaders yesterday, Chairman Charles H. Percy (R-Ill.) of the Senate Foreign Relations Committee described the proposed program as being "as important as the Marshall Plan was" in rebuilding Europe following World War II.

Despite Percy's praise, the program, which the administration hopes to link with parallel aid efforts by Canada, Mexico and Venezuela, is expected to provoke controversy. Many Caribbean-area governments have already complained about lack of consultation and have expressed concern that the program will be inadequate for their needs and tied too closely to U.S. ideological concerns such as fear of Cuban influence.

The broad outlines of the plan, under preparation since last spring, call for an immediate increase of approximately $300 million in financial assistance to the region, preferential tariff arrangements for its products such as sugar and rum, and incentives for U.S. firms to make job-creating investments in the area.

Much of the proposed program will require congressional approval, and there is concern in the administration about resistance from business and labor groups opposed to special preferences for the Caribbean region. In addition, many congressional liberals, while approving the general thrust of the program, are concerned that it not become merely a vehicle for pursuing the administration's campaign against leftist guerrillas in El Salvador.

In recent days, there has been a tug of war between some administration officials who want the speech to concentrate on economic issues and Secretary of State Alexander M. Haig Jr., who advocated including tough language about communist penetration of the region.

According to administration officials, the speech, while putting its first emphasis on economics, probably will contain a new warning about Reagan's resolve to combat alleged Cuban subversion in Latin America and a reaffirmation of U.S. support for the civilian-military government in El Salvador.

Congressional arguments about the wisdom of the administration's Central America policy continued yesterday as Elliott Abrams, assistant secretary of state for human rights, appeared before a House subcommittee to defend Reagan's recent certification that Salvadoran authorities are working to prevent rights abuses and thus are eligible to continue receiving U.S. military aid.

Abrams told the subcommittee on international organizations that the Salvadoran government is trying "to control the level of violence" and ensure that the country carries out free elections for a constituent assembly scheduled for next month. The Salvadoran guerrillas are boycotting the elections, and the administration yesterday reacted with a coolness bordering on rejection to Mexican President Jose Lopez Portillo's weekend offer to try to mediate between the contesting factions.

House Speaker Thomas P. (Tip) O'Neill Jr. (D-Mass.) said yesterday he believes there should be negotiations with the guerrillas following the March 28 elections. O'Neill also supported continued U.S. economic aid to El Salvador, but, drawing a parallel with Vietnam, warned that Congress will not consent to sending U.S. troops to help the Salvadoran government fight the guerrillas.