The retrial of two anti-Castro Cubans accused of murder in the 1976 assassination of Orlando Letelier drew to a close in U.S. District Court yesterday as prosecution and defense lawyers focused their final arguments on Michael Vernon Townley, the key government witness who testified that he organized the plot to kill Letelier.

Assistant U.S. Attorney E. Lawrence Barcella Jr. told the jury that telephone and travel records, an address book, letters, receipts, other documents and components to explode a bomb, all of which were obtained by federal investigators in the case, support Townley's testimony that he recruited the Cubans to help him carry out the murder on orders from the Chilean secret police, once known as DINA.

Defense lawyers Paul Goldberger and Lawrence A. Dubin described Townley, an American-born DINA agent, as a polished liar who, still acting on orders from DINA, cooperated with U.S. prosecutors only after he was instructed by Chilean officials to implicate the Cubans. The defense contends the assassination was carried out by Townley, DINA and Chile alone.

Letelier, a former Chilean ambassador, was a vigorous, outspoken opponent of the military regime of Augusto Pinochet, which took over the Chilean government during a bloody coup in 1973. Townley testified that Chilean military officers told him Letelier had to be killled and ordered him to have the Cubans carry out the plot to insulate the Pinochet government from any culpability.

Townley, who was expelled from Chile in 1978 and turned over to U.S. prosecutors, has pleaded guilty to conspiracy to murder a foreign official and is serving a sentence of 3 1/2 to 10 years.

Letelier and an associate, Ronni Karpen Moffitt, were killed on Sept. 21, 1976, when a bomb exploded under Letelier's car as it rounded Sheridan Circle on Embassy Row. The two defendants, Guillermo Novo Sampol and Alvin Ross Diaz, were charged with murder and conspiracy along with two other Cubans, both of whom are fugitives. Novo and Ross were convicted of murder and conspiracy charges in 1979, but they were granted a new trial on appeal. Three Chilean military men, including Juan Manuel Contreras Sepulvada, once the head of DINA, also were indicted in the murders, but the Chilean courts refused to extradite them.

The prosecution contends that Chilean officials, under intense pressure from the United States in connection with the Letelier case, told Townley to tell the truth about the killings.

But defense attorney Dubin told the jury of eight women and four men that the prosecution "needed some bodies" to try. Townley, whose wife and children still live in Chile, went along with the ground rules set up by the Chilean government so that he could aqssure his return ho his family and way of life there, Dubin said.

The jury is expected to begin deliberations today.