A Cuban exile charged with conspiring to murder former Chilean ambassador Orlando Letelier was ordered held here yesterday on $250,000 bond after a prosecutor described him as having "substantial knowledge" about the alleged murder plot.

Ignacio Novo Sampol, 39, of Jersey City, N. J., a leader of the Cuban Nationalist Movement, was arrested Friday morning under a still-secret arrest warrant. Yesterday's court proceeding before U.S. Magistrate Lawrence S. Margolis was for setting bond here.

Assistant U.S. Attorney E. Lawrence Baracella Jr. of the major crimes division said the case against Novo is a "solid one" and asked for a high bond.

Barcella said the government feels it can prove that Novo had "full, complete and substantial knowledge of the facts surrounding the murder" and that he lied when he testified before a federal grand jury here.

Novo had "prior knowledge" of the assassination and knew how it would be carried out, Barcella added. He allegedly took several steps to see that the plot was successful, Barcella said.

Letelier, an outspoken critic of the military government now ruling Chile, was murdered here Sept. 21, Circle NW. A bomb placed under the front seat of his car ripped through the floorboards, killing him and Ronni K. Moffitt, an aide at the Institute for Policy Studies.

Barcella said that, in addition to the two federal charges lodged against Novo, he could face additional local and federal charges for his alleged role in the two murders.

Barcella argued that Novo might attempt to flee if he is released from jail, and said there is evidence that others believed involved in the murder - including Novo's brother, Gullermo Novo Sampol - had false travel documents and disguises.

Guillermo Novo was arrested on a fugitive warrant in Miami las month after failing to show up for a court appearance in New Jersey last June.

Defense attorney William Garber said he had no way of evaluating the strength of the government's case against his client because the papers are still sealed. But he said that Ignacio Novo had shown up when requested in the past.

He described Ignacio Novo as an unemployed shoe and auto salesman who lived in New York for about 20 years before moving to Miami four years ago. Novo moved back to New Jersey about a month ago, according to court records.

Garber said in arguing for a "reasonable" bond that Novo could have fled before his arrest last week because he was aware he might be charged in the assassination plot.

Novo is among six persons who have been identified as being involved in the alleged murder plot.

Federal officials are reportedly confident that they can trace the murder from its origins in Chile as a project of the Chilean secret police known as DINA to its excution by anti-Castro Cuban exiles sympathetic to the Chilean military government.

The person who reportedly acted as the contact between DINA and the American-based Cubans - and American-born DINA agent named Michael V. Townley - is cooperating with U.S. authorities as part of a plea-bargaining arrangement, sources have said.