Leaders throughout the Caribbean Basin gave a generally warm welcome to President Reagan's long-awaited economic initiative for their region yesterday, but the U.S. plan came under scathing criticism from the Soviet Union and Cuba, which Reagan blamed for spreading instability in the area.

From Jamaica to El Salvador, countries that are likely to gain aid and economic concessions under the U.S. plan greeted it as a powerful boost for the area's sagging economies. The plan would grant $350 million in economic assistance and $60 million in military assistance to U.S. friends in the region.

The Soviet news agency Tass, however, denounced the military and financial package, presented to the Organization of American States Wednesday, as an attempt to shore up "blood-stained repressive regimes" trying to strangle by terror the desire of the region's people for freedom and democracy.

And in the latest of a series of critical Chinese press commentaries on U.S. foreign policy, the official New China News Agency said Reagan's program showed his "determination to intensify U.S. rivalry with the Soviet Union for hegemony in the region."

Mexico, which with its oil wealth is a powerful spokesman in the Caribbean and which had previously indicated it would join the United States in a coordinated economic aid plan for the area, was reserved in its initial comments. Foreign Minister Jorge Castaneda told The Associated Press, "We thought . . . the part dealing with economic assistance to the Caribbean . . . was constructive and useful. As for the political aspect, I thought it was rather strong."

Mexican President Jose Lopez Portillo steadfastly has opposed foreign military intervention in Central America and the U.S. effort to defeat leftist expansion in the region. Sunday, he offered to mediate the conflict in El Salvador.

Reagan's address, which outlined the aid package as well as a plan to drop tariffs on all Caribbean products except textiles, singled out Cuba--and nations he said it influences, such as Grenada and Nicaragua--for undermining the region's stability. Radio Havana lashed back yesterday, calling Reagan's speech "a mixture of lies, cynicism and threats" and the total amount of the aid "ridiculous."

"In truth, the so-called aid is only an attempt to end the economic deterioration and political loss of prestige of the OAS, an organization subordinate to U.S. imperialist interests," the commentary said.

The official Cuban news agency Prensa Latina said the U.S. plan was an attempt to involve the countries in the politics of cold war and was meant to assert economic dominance over them, The AP reported. Prensa Latina added that the proposal was a new attempt to isolate Cuba, Nicaragua and Grenada from the rest of Latin America.

The Soviet press echoed the same arguments. Tass said the United States was planning to "destablilize" the progressive governments in those three countries and ridiculed Reagan's assertions that Soviet and Cuban weapons were being channeled to leftist guerrillas in El Salvador, Washington Post Moscow correspondent Dusko Doder reported.

Although there was no immediate comment from Managua, Nicaragua's ambassador to the United States, Francisco Fiallos Navarro, said on the television show "Morning" (CBS, WDVM) that he thought the Reagan plan was "a good step the region needs badly" and that Nicaragua would like to be included in the plan.

French President Francois Mitterrand, who has tried to maintain strong ties with the Sandinista government in Nicaragua, said he generally approved of the Reagan initiative, Washington Post correspondent Jonathan C. Randal reported.

"This initiative is not exactly what France would have done," Mitterrand said, "but it is going in the right direction."

Much of the proposed aid would go to El Salvador, Costa Rica and Jamaica, which responded enthusiastically. Salvadoran President Jose Napolean Duarte called the plan unprecedented and said it "assures the success of our political project," an apparent reference to elections scheduled there next month.

In Jamaica, Prime Minister Edward Seaga described the initiative a "bold, historic and far-reaching."

Costa Rican President Rodrigo Carazo called the plan "historic and transcendental."

Panama's foreign minister, Jorge Illueca, called the plan encouraging but said "Latin America's problems cannot be solved with military aid, whether from the Soviet Union, Cuba, Nicaragua or the United States."