A Bethesda firm that serves as a broker in the international arms market has been indicted by a federal grand jury on charges of selling military equipment to the Chilean government in violation of U.S. export laws.

The 21-count indictment against United Aviations Industries Inc. and its two principal officers were said by federal officials to be part of a broader investigation into efforts by the regime of Gen. Augusto Pinochet to circumvent an American arms embargo imposed in 1976 after reports of widespread human rights abuses there and Chilean complicity in the murder of Orlando Letelier, the country's former ambassador to the United States.

The indictment charges that United Aviations President Robert Poisson of Chevy Chase and Vice President Anthony Villa of Oxon Hill conspired with two members of the Chilean naval mission in Washington to disguise the shipment of parts for torpedoes, launchers, depth charges, guns and sonar equipment. The shipments were "destined for the Chilean navy," according to the indictment handed up Monday by a grand jury in Norfolk.

Alfred Swersky, lawyer for United Aviations and Poisson, said yesterday that the Chilean government "is and was a customer" of the company, but said that his clients have violated no laws. Bryan P. Gettings, a lawyer for Villa, said his client also denied wrongdoing.

He estimated that the total value of the arms shipments was not more than $50,000. "This is not about a high technology drain to the Russians," Gettings said. "This is peanuts."

Jerry Friedlander, a Washington lawyer for the Chilean government, said his client would have no comment until it has an opportunity to review the charges. The two Chilean navy officers, Capt. Jorege Acuna and Cmdr. Douglas Ashcroft, both of whom had headed the naval mission's purchasing department, were named as unindicted coconspirators. Both are in Chile, Friedlander said.

United Aviations was described by Swersky and others yesterday as a small international arms broker that arranges sales between foreign government purchasers and U.S. weapons suppliers. Swersky said the firm consisted of only five employes, "three of whom are relatives." In addition to Poisson and Villa, he said, the employes include Villa's wife, Villa's daughter and a secretary.

The investigation into United Aviations grew out of Operations Exodus, a Reagan administration program to crack down on the illegal export of critical technology, principally to the Soviet Union and its allies. Recently, officials said, the Customs Service and other agencies involved in Exodus have turned up evidence of Chilean arms purchases.

Late last year, the U.S. attorney's office in Alexandria seized documents relating to Chilean weapons shipments after searching the offices of the Chilean air force mission, which was renting space from an exporting firm in Arlington, officials said yesterday.

The indictment outlines a complicated series of alleged transactions by United Aviations and the Chileans intended to disguise prohibited weapons shipments that were executed through a variety of suppliers, exporters and middlemen. The alleged purchases were begun by the two Chilean navy officers in February 1977, shortly after the embargo went into effect, and continued until July 1982, according to the indictment.

Because the military items were banned by State Department for export to Chile, the indictment charges that United Aviations furnished false or misleading information about the shipments on export declarations.

The indictment specified how one such shipment allegedly occurred. United Aviations ordered a $25,000 purchase of sonar equipment from the Ford Aerospace and Communications Corp. in Norfolk on May 5, 1981. The cargo was shipped from Norfolk to a Northern Virginia exporting firm. It was seized by customs officials at John F. Kennedy International Airport in New York while being loaded onto a Lan-Chile Air Cargo plane on May 27, 1982. The sonar parts had been listed on shipping documents as aircraft parts.