Mexico presented the United States today with details of its proposal for a regional political solution to the growing confrontation in Central America through "an exchange of mutual concessions" between the United States and leftist nations and forces in the area.

According to Mexican Foreign Minister Jorge Castaneda, who gave the plans to Secretary of State Alexander M. Haig Jr. in a 2 1/2-hour meeting, Haig did not reject anything out of hand. The Mexican indicated that he expects a reply, although not necessarily a final or definitive one, from Haig in another meeting here sometime next weekend.

Today's session was the diplomatic follow-up to the peace proposals made public by Mexican President Jose Lopez Portillo two weeks ago in Managua, Nicaragua.

The first U.S. reaction to the Mexican proposal was cool, making the point--as Haig did again today--that it did not explicitly address the question of Nicaraguan support for insurgent forces waging war inside El Salvador.

Haig, calling this point "an essential and primary element" of a solution, indicated that he asked Mexico today to add Nicaraguan nonintervention to the list of its principal proposals.

For his part, Castaneda agreed, in a press conference immediately after Haig's, that the issue of outside arms in El Salvador is "one of the most important ones and it has to be solved."

Castaneda went on to say that "it has to be settled in a wider context, in a system of trade-offs" between the United States and regional powers such as Cuba and Nicaragua.

The Mexican view is that the Salvadoran problem is only one dangerous outcropping of a general crisis throughout Central America and that efforts to solve it without addressing the larger context are not likely to succeed.

Castaneda did not elaborate on what U.S. concessions would be expected as part of such trade-offs, except to say that assurances of U.S. military restraint and "an end to verbal terrorism" would be among the elements.

In announcing his proposal Feb. 21, Lopez Portillo called for:

A renewal of dialogue and a "system of mutual concessions" by the United States and Cuba.

U.S. disavowal of "any threat or use of force directed against Nicaragua"; the disarming and suppression of anti-Sandinista bands along the Honduran-Nicaraguan border and in the United States in return for a renunciation of new armaments by the Nicaraguan government, and a system of mutual nonaggression pacts between the United States and Nicaragua on the one hand and Nicaragua and its neighbors on the other.

A compromise solution through negotiations in El Salvador which satisfies the principal concerns of the United States.

Castaneda said additional details of these proposals were supplied to Haig in the meeting today.

Haig said it is "too early to say" whether a negotiated solution is possible. But he said the Reagan administration is committed to explore every avenue, including "exploring the Mexican initiative in depth."

It was clear from Castaneda's remarks that he has little expectation of movement in such negotiations before the Salvadoran constituent assembly elections scheduled for March 28.

In Castaneda's view those elections are "worth nothing," because no arrangements have been made for the participation of the guerrilla forces and their supporters. Nevertheless, he recognized that no major changes of direction can be expected at present in view of the uncertainty surrounding the outcome of the balloting.

It is clear both to him and Haig, the Mexican said, that eventually U.S.-Cuban arrangements and U.S.-Nicaraguan arrangements "are likely to have an effect" on the increasingly tense and bloody situation in El Salvador.

As for the arms flow into El Salvador from outside, Castaneda said Haig had not given him the much-heralded evidence in possession of the United States. In the Mexican view, he said, the flow of arms is "fairly small and unimportant" now, although he added that it might have been greater at some point in the past.

Under questioning by reporters Haig said publication of some of the "irrefutable evidence" of outside intervention in El Salvador is likely this week, perhaps Tuesday or Wednesday. Haig said he had reviewed a "dry run" public briefing on the subject Friday but he believes it needs "some improvement."

The two ministers sought to avoid detailed discussion before reporters of the controversial Nicaraguan "guerrilla" or "student" who took refuge in the Mexican Embassy in El Salvador. Haig refused to answer any questions about the Nicaraguan, whose capture he announced in a House hearing Thursday, except to reiterate the statements he made at the time. Castaneda said Mexico had given the man political asylum on the basis of his claims of being a student in Mexico but had no independent evidence.

While the American and Mexican foreign ministers explored the problems of Nicaragua and other Central American states over lunch, one of the senior Nicaraguan leaders, Jaime Wheelock, was in the same hotel nine floors below. Castaneda plans to confer with Wheelock here this weekend, but there are no plans for Wheelock to meet with Haig.