Secretary of State Alexander M. Haig Jr. and Mexican Foreign Minister Jorge Castaneda will meet in New York Saturday to discuss a broad peace initiative for Central America proposed by Mexico's president, officials of both countries announced today.

During a private lunch, Castaneda is expected to try to convince Haig of the usefulness of the Mexican proposals and to persuade Washington to seek negotiations to solve the Salvadoran conflict.

Mexico, which is understood to have requested the meeting, has offered to act both as a "communicator" in the proposed talks and as a guarantor of any outcome.

Castaneda, who already was scheduled to go to New York to attend the Law of the Sea conference, said in an interview before leaving today that Mexico was "very happy with the broad support we received in European nations and even sectors in the U.S." for the initiative launched by President Jose Lopez Portillo in Managua on Feb. 21.

The Mexican plan calls for separate talks between the United States and Cuba and between the United States and Nicaragua, and for peace negotiations among all the parties involved in the conflict in El Salvador.

Castaneda, giving no details, said he would present Haig with "concrete points for trade-offs and mutual concessions" to deal with the knots of tension in the region.

On El Salvador, Castaneda is expected to urge Haig to create alternatives beyond this month's elections for a constitutional assembly.

"The U.S. has put 90 percent of their marbles on elections," Castaneda said, "but there is a need for other solutions." If the elections fail, "what will the U.S. do with that hot potato?" he asked.

"The main problem," Castaneda said, is "that the U.S. is painting itself into a corner, which may force it to resort to extreme measures. It is the duty of people who want peace in Central America to prevent the U.S. from cornering itself."

Mexico has been increasingly concerned about widening of the Central American conflicts, and it fears that current Washington policies are leading to the radicalization of the outcome of any settlement.

Announcing his initiative in Managua two weeks ago, Lopez Portillo stressed that the "fundamental security interests" of the United States need not be endangered by a process of detente in the region. "Mexico and other friends and allies of the U.S. would be in a position to provide guarantees on this point," he said.

Although Mexico has become more involved in the politics of the region during the past three years, this is the first time it has been willing to assume a direct--albeit unspecified--responsibility for preserving its stability.

Asked why Mexico was adopting this role, Castaneda said, "We believe that whatever happens in Central America affects us...and we are the only country in the region that has excellent relations with Cuba, Nicaragua and the United States. We have contacts with the opposition in El Salvador and diplomatic relations with the junta."

Castaneda said that several European governments had either approached Mexico or expressed strong support for the initiative. He would not name them.

Yesterday, the Italian parliament followed the example set by Mexico and France in August in recognizing the Salvadoran opposition as a "representative political force."

The key to regional detente, Lopez Portillo is believed to feel, lies in a rapprochement between Havana and Washington, and Mexico looking for ways to follow up on the meeting here in November between Haig and Cuban Vice President Carlos Rafael Rodriguez, which Mexico engineered..

Officials here stress that Mexico is anxious to avoid a radical outcome in El Salvador. While popular revolutions are necessary in Central America, Castaneda said, they "need to be channeled within reasonable limits."

On Nicaragua, Mexico has been more specific, calling for the Sandinista government to reduce its arms buildup and renounce acquisition of Mig jets and other war materiel, in exchange for a disarming of Nicaraguan exiles operating out of Honduras and training in the United States.

Mexican officials said that in private talks with Sandinista commanders in Managua last month, Lopez Portillo was told that Nicaragua still felt the need to acquire Mig jets because its neighbors, Honduras and El Salvador, possessed sophisticated aircraft and armaments, but that it would accept a balanced reduction of arms throughout the region.