Mexican President Jose Lopez Portillo, decrying what he called "the new climate of cold war" and indirectly putting much of the blame for it on the Reagan administration, today sharply underlined his growing foreign policy differences with Washington, condemning the neutron bomb and clashing directly with the United States over much of its Caribbean Basin policy.

In his wide-ranging, fifth annual State of the Nation address, Lopez Portillo talked of the need for the United States and Mexico to "transcend their traditional mold and look for shared criteria and solutions."

He expressed hope that destabilizing economic problems might be resolved by the joint aid programs being discussed by Mexico, Venezuela, Canada and the United States and by the conference in Cancun in October for the leaders of 22 rich and poor nations.

But the Mexican president reiterated positions on worldwide issues and regional questions that put him in conflict with policies of the Reagan administration.

Condemning the demise of the second Strategic Arms Limitation Treaty, which the United States has failed to ratify, Lopez Portillo said, "Everything indicates that this longed-for agreement has now been replaced by the development, construction and warehousing of the worst that mankind has produced: the ominous neutron bomb."

Turning to regional policy, Lopez Portillo emphasized, to resounding applause from the Mexican congress, his government's support for Cuba, Nicaragua and leftist insurgents in El Salvador.

"Over the last year we have centered our action on the nearest area, geographically and politically, to our own essence -- Central America and the Caribbean -- holding high the banner of nonintervention," Lopez Portillo said. "We have, on repeated occasions, made clear publicly, privately and in many ways, our disagreement and opposition to all types of interference in the area, especially by the superpowers.

"By further tightening the links of friendship and cooperation that bind us with the revolutions of Cuba and Nicaragua, we have underscored Mexico's attachment to the political principle of a people's free determination. For sympathy and affinity with the essence of their struggle -- social justice -- Mexico has helped them and will continue doing so."

The Reagan administration has portrayed the Salvadoran civil war as an example of thinly veiled aggression by the Soviet Union.

For most of this century Mexico has been a relatively withdrawn figure on the international scene. But as its oil wealth has grown, so has its involvemement in international affairs, especially in Central America and the Caribbean.

Mexico is now the fourth greatest oil power in the world.

Lopez Portillo noted in his speech the discovery of even greater petroleum resources than previously announced, totaling 250 billion barrels in potential oil and gas reserves, with a 6 percent increase in proven reserves during the last five months from 67.83 billion barrels to 72 billion barrels.