Vice President Walter Mondale told Mexico today that there would be no massive deportations or illegal aliens from the United States and offered to throw U.S. weight behind a massive multilateral finance program to curb the flow of illegal Mexican migrants.

Mondale, who is on a three-day official visit here, made his remarks at luncheon after meeting with Mexican President Jose Lopez Portillo for nearly two hours.

The details of the finance program were not disclosed, but Mondale said that the United States is "ready and eager to do all it can, including support expanded efforts by the World Bank and the Inter-American Development Bank to increase rural development efforts in Mexico." The two institutions, he said, had already agreed to significant loans.

The program, as officials described it, would involve large investments to create jobs in the depressed agricultural areas of northern and centraal Mexico from which several million workers cross illegally into the United States every year.

In response to Mexico's often-expressed concern about the treatment of its citizens in the United States, Mondale today said in his speech that "there will be no massive deportation or roundup."

Mexico, which is in the midst of a serious economic crisis, strongly opposes the Carter proposal to put a stop to all illegal migration to the United States, which would cut off a vital escape valve for Mexico's nearly eight million unemployed.

Mondale stressed in his airport statement on arrival and in his speech today that this has been a year of "unparalleled accomplishments" in U.S. Mexican relations.However, the two most important issues between the two countries - illegal migrants and Mexican sales of natural gas to the U.S. - are expected to remain unsolved during this visit.

Last month, negotiations with six U.S. companies over the sale of Mexico's gas surplus broke off as Energy Secretary James Schlesinger refused to approve the Mexican price of $2.60 per thousand cubic feet. It was too high compared to the $2.16 paid to Canada, Mexico was told.

Although Mexican officials now say they are fully aware that an agreement on the Mexican gas price hinges on natural gas legislation now before Congress, the U.S. refusal has caused considerable ill feeling here.

The vice president said he had raised the U.S. Mixican energy program with the Mexican president but disclosed no further details of their conversation.