Two anti-Castro Cuban exiles, whose murder convictions for the 1976 car bombing assassination here of former Chilean ambassador Orlando Letelier were overturned last year, were ordered released from jail yesterday after a group of six Cuban expatriates raised $400,000 in cash to meet their bond.

U.S. District Court Judge Barrington D. Parker approved the release over the objections of Assistant U.S. Attorney E. Lawrence Barcella Jr., who voiced fears that the two defendants, Guillermo Novo Sampol and Alvin Ross Diaz, would flee before their retrial, scheduled to start May 11, is completed.

The judge said Novo and Ross could live with friends and relatives in Union City and Jersey City, N.J., but said he he would issue a court order strictly limiting their travel to that northern New Jersey area and to their attorneys' office in New York City.

The six Cuban businessmen, including a bank president and owners of a jewelry store, a meat plant and a clothing store in northern New Jersey, had formed a "Committee for the Bail of Novo and Ross" and collected the cash in a three-week period last month. Five of the six men on the committee, except the bank president, all of whom were present in the courtroom yesterday, got $20,000 mortgages on their homes, collected another $15,000 from a similar committee in the Miami area and solicited the remainder in contributions from friends and relatives of the defendants.A sixth man, not on the committee, also took a $20,000 mortgage on his home.

The businessmen said they have deposited the $400,000 in a Union City bank and paid another $18,000 to a bonding company to take the case.

Defendants routinely post money bonds to secure their release in the District of Columbia, using cash and other collateral, usually property. The case yesterday was viewed as unusual because the extraordinarily high bond for both Novo and Ross was totally covered with cash, eliminating any risk to the bonding company should the two men fail to appear in court.

In brief interviews yesterday, the men described themselves as anti-Castro, anti-communist Cuban refugees who are interested in the Letelier case. "We believe they are innocent. That's the main reason" the money was raised, said Mario Fernandez, a sales manager for Prudential Insurance in Newark, N.J., and a member of the committee.

Letelier and a young associate, Ronni Karpen Moffitt, were killed Sept. 26, 1976, when a bomb exploded in their car at Sheridan Circle along Embassy Row. During a six-week trial in January and February of 1979, the government's key witness, Michael Vernon Townley, an American-born agent for the Chilean secret police, testified that he recruited Cubans, including Novo and Ross, to carry out the killing. The government contended that the secret police had orchestrated the murder of Letelier, who in exile had been an outspoken critic of the military dictatorship of Gen. Augusto Pinochet.

Parker said he expects that the two men could be released from jail today, when he completes his final release order. Barcella also asked Parker to require both men to turn over any travel documents they have, such as a passport, and to prohibit them from entering any foreign embassy or consulate in an effort to obtain santuary from prosecution. The two men also will be required to report regularly to the U.S. marshal's office in Newark.

Parker early last month said both men could be released if they each posted $200,000 surety bonds. A surety bond means that Novo and Ross would have been required to get a bondsman to back up their bond with his promise that the two men would appear for all further court hearings in the case. Courts routinely require that a bondsman be involved in a release order if a judge believes there is a risk that the defendant will flee while on bond.

Usually, a bondsman charges a fee or premium to take a case, accepts some part of the bond in cash from defendants and the remainder in some form of collateral, such as the deed to property. The theory is that bondsmen, by taking some financial risk in a case, thus have a stake in seeing that a defendant appears in court. If a defendant fails to appear, the bondsman must forfeit the full bond to the court.

In yesterday's case, however, Barcella protested that the bondsman for Novo and Ross face no risk sine the entire $400,000 is on deposit in a Union City bank in the form of certificates of deposit that have been assigned to the bonding company, Nationwide Bail Bonds Inc. of Passaic, N.J. If the two men fail to appear in court, Barcella said the company would simply turn the money over to the court with no loss to it. The company already has collected the $18,000 premium.

The $120,000 in mortgage money to the six men was issued by the Pan American National Bank of Union City, whose president, Luis Perez Vega, is a member of the committee, sources said yesterday. Vega made a past contribution to the committee, these sources said. The noncommittee member who got a mortgage from the bank was Novo's brother-in-law.