Newly installed Peruvian President Alan Garcia declared a two-day bank holiday today that financial sources said was a precautionary step to prevent a run on international reserves and perhaps a prelude to exchange controls.

Police and bank inspectors were sent to all financial institutions and money exchange offices to enforce the ban on financial operations.

At a press conference, Garcia refused to explain the purpose of shutting the banks, saying, "Policies are not announced, they are implemented." He added that "capital flight has already occurred, even without exchange controls."

In another development, six Latin American heads of state and representatives of 14 other regional governments here for Garcia's inauguration last Sunday issued a document of sweeping generalities called the Declaration of Lima.

The subject of considerable negotiation, this declaration emerged as an apparent update of a Consensus of Cartagena, which was drafted by 11 Latin American governments a year ago in that Colombian city. The Declaration of Lima calls for a new round of meetings, starting next month.

The declaration offered rhetorical support for the Garcia administration but not for its specific refusal to deal with the International Monetary Fund or to use more than 10 percent of export income to service the foreign debt.

Carlos Franco, one of Garcia's closest political advisers, said, "There is no possibility of militant solidarity from Latin America for the time being. We know we're going to be standing in the cold alone on the issue."

The most immediate question for the Garcia administration is whether it has the technical expertise to put into action the promises made in Garcia's strongly worded inaugural speech.

His party, the 60-year-old American Popular Revolutionary Alliance (APRA), never before has held power. Premier and Finance Minister Luis Alva Castro is a politician not an economist.

In a related development, Sam Dillon of Knight-Ridder reported from Havana:

President Fidel Castro, apparently frustrated over the lukewarm response to his proposal for erasing Latin America's foreign debt, has sent an acidic letter to Peruvian President Garcia questioning his sincerity as a social reformer.

"It is the most insulting diplomatic message I've ever seen," said a European diplomat after the Cuban Communist Party daily Granma published the letter Monday, on the eve of a Havana conference on regional debt.

Garcia, with his center-left credentials, is regarded by his aides as a possible challenger to Castro's leadership of the Latin American left. He vowed at his presidential inauguration Sunday to limit Peru's foreign debt payments but called the Havana conference a bad approach to the crisis.

Earlier this month, Garcia criticized Castro's proposal as an attempt to convert the debt crisis into an "East-West struggle" and called it "interesting but unrealistic."

Garcia said Latin countries, while negotiating with their creditors for better terms, should continue trying to meet their obligations.

Castro's letter said Peru was "one of the Latin American countries with the highest levels of illiteracy, infant mortality, disease and lack of medical attention, malnutrition, unemployment, social inequality and misery of every type, in spite of its immense natural resources.

" . . . If you decide, really, to fight seriously and firmly against this hellish picture of social calamities . . . as you have publicly promised . . . you can count on Cuba's support," Castro wrote.

The letter was regarded as the bluntest evidence of Castro's irritation at Latin leaders who have failed to respond to, or have criticized, his debt proposals.

No Latin country appears to have sent senior government officials to the Havana talks that started late Tuesday. Castro has said it was not a diplomatic summit, but he spent months urging leaders to attend.

The conference has no agenda and will consist of hours of speech making by the ex-officials, intellectuals and leftist politicians.