Key government evidence in connection with the bombing assassination of former Chilean ambassador Orlando Letelier was seized last March by FBI agents without a search warrant, even though an agent said it was necessary to have one, a Union City, N.J., building manager testified here yesterday.

Luis Vega, the building manager, said he gave the FBI permission to take various materials from an office. The government contends that the materials were used to help make the bomb that killed Letelier and a colleague, Ronni K. Moffitt, in Letelier's car on Sheridan Circle Sept. 21, 1976.

But Vega testified that when he first offered the materials to FBI agents last Feb. 28, an agent told him that a search warrant was needed because it was "just as if (the property) belonged to" a man Vega knew as Carlos P. Garcia who had rented the office.

On March 6, the agents took the materials, with vega's permission but without a search warrant. The materials included hollowed-out shotgun shell casings, an electric cable and a sales receipt for electronic paging equipment that the government contends was used to detonate the bomb attached to Letelier's car.

Government prosecutors are contending that "Garcia" actually is Alvin Ross Diaz, a Cuban exile on trial in U.S. District Court here for the murder of Letelier and Moffitt. Two other Cuban exiles, Guillermo Novo Sampol and his brother, Ignacio Novo Sampol, are being tried with Ross.

Vega identified yesterday a pciture that he was shown by FBI agents last Feb. 28 as the man he knew as Garcia. Despite three visual searches in the courtroom, Vega said he could not positively identify the man he knew as Garcia.

The government has argued that Ross abandoned the office by not paying rent after Oct. 30, 1977, more than four months before Vega turned over the papers and materials to the FBI.

Vega testified in connection with a defense bid to suppress evidence. Judge Barrington D. Parker spent most of the trial's third day questioning prospective jurors.