Mexican Finance Minister Gustavo Petricioli told the world's leading commercial bankers here tonight that his debt-ridden Latin nation needs $12 billion in new loans to get through 1987, and asked them to supply half of it.

The money will avoid deepening Mexico's current recession and "allow us to resume moderate but sustained growth in 1987," he told a meeting of bankers who already hold 80 percent of the country's $97.6 billion debt, the second largest in the developing world.

"For 19 months," he said, "we have punctually paid interest without receiving any credit, either in cash or public opinion. Through devastating earthquakes and just as devastating drop s in oil prices, we have been willing to take the necessary adjustment measures and continue to honor our commitments.

"Any realistic analysis would conclude that Mexico cannot continue to do so without the appropriate external support."

He tried to reassure the bankers that they would not be throwing good money after bad, saying that the President Miguel de la Madrid's revisions will spur the economy with the help of new loans.

Addressing a sensitive subject, Petricioli said capital flight from the country has been stemmed. He said the government's interest and exchange-rate policies and a tight money policy produced an inflow of $1 billion in new capital during the first four months of 1986.

Petricioli, who took over as finance minister last month, indicated that Mexico may ask for some concessions from the banks, but he suggested that lowering interest rates might be an "intelligent recognition of the fact that the debtor needs real relief in order to service its debt regularly." He said it would be "an act of economic rationality, which is long overdue" to link debt payments to oil prices, since most loans were made between 1977 and 1982, when Mexico's future appeared rosy because of its oil wealth.

Petricioli's pitch to the bankers, who had received no details on Mexico's plans to get out from under its massive debt, was strongly endorsed here by the heads of the International Monetary Fund and the World Bank, both of which announced $3.6 billion in new loans to Mexico Tuesday as their share of the rescue operation.

"I am one of the exhorters," said World Bank President Barber B. Conable in an interview today.

He said the World Bank intends to loan Mexico another $1.5 billion next year on top of the $2 billion in loans announced Tuesday for 1986 as part of its increased role in the Mexico rescue operation, and added that should help reassure commercial lenders that their money is safe.

Bankers leaving the three-hour presentation generally refused comment. Some, who did not want their names used, said the Mexicans had not been specific enough on their economic-revision program for them to make any decisions.

Petricioli was quoted by some bankers as saying he would like an answer on the new loan request by September, when the IMF meets.

Bankers have indicated concerns that the economic revisions that are the linchpin of the new IMF commitment might evaporate under the political pressure of an election campaign in two years.

These revisions would seek to reduce Mexico's budget deficit, liberalize its protectionist trade practices, restructure its tax system, allow more foreign investment, cut subsidies and transfer public-sector companies to private interests.

"We are impressed by the magnitude of the adjustment efforts already under way in Mexico . . . " Conable told the bankers.