A Mexican newspaper today published a facsimile of a letter purportedly signed by the head of Chile's secret police requesting more government funds for its activities in "neutralizing" its principal opponents in seven countries, including the United States.

A carbon copy of the text, dated Sept. 16, 1975, and addressed to Chilean President Augusto Pinochet, was sent anonymously to Hugo Miranda, an ex-senator in the Chilean Radical Party who lives in Mexico City.

Miranda said the copy was handed him by a contact that he would not name. "I am convinced that the document is authentic because it is from an impeccable source," he said, adding that it was not the first "secret military document" that the exiles have received.

The authenticity of the copy has not been established.

The facsimile, as published in the newspaper El Sol, is signed "Manuel Contreras Sepulveda, Colonel, Director of National Intelligence" (DINA). It says, "Further to the agreement with Your Excellency, I specify the reasons why I consider it indispensable to apply for an increase above the $600,000 in this year's budget."

Those reasons listed are:

"1. An increase in the DINA staff attached to Chile's diplomatic missions. A total of 10 persons: two in Peru, two in Brazil, two in Argentina, one in Venezuela, one in Costa Rica, one in Belgium and one in Italy." (Apparently the request is for 10 additions to an existing staff.)

"2. Additional expenses for the neutralization of the principal opponents of the government junta abroad, especially in Mexico, Argentina, Costa Rica, United States, France and Italy.

"3. Expenses related to the operations in Peru: Aid for our supporters in the Peruvian Navy and in the press, particularly contributions to 'Equis-X' and 'Opinion Libre.'

"4. Funds for the officials of Directorate who attend courses in preparation for anti-guerrilla groups in the training center of the city of Manaus, Brazil."

On the lower half of the page the letter is stamped with the emblem of "DINA, Presidency of the Republic."

The publication has created further consternation in Mexico. In recent months, threats have increased here against prominent exiles who were members of former President Salvador Allende's government.

Some Chileans have moved away from Mexico, and others have received the Mexican government's assurances that its own secret police are investigating and watching the conduct of Argentine and Chilean agents found to be living here.

Last weekend the Mexican government received information that terrorist actions were being plotted against members of the South American exile community who fled rightist repression at home.

The publication of the letter comes against the background of indications given by U.S. federal investigators that the bombing death of former Chilean Foreign Minister Orlando Letelier in Washington in September may have been directed by persons in Chile.

It also follows new evidence gathered by Chilean exiles that the 1974 bombing death of the former Chilean army chief, Gen. Carlos Prats in Buenos Aires, was carried out in close cooperation with the persons at the Chilean embassy in Argentina.