The D.C. Court of Appeals ruled yesterday that a witness cannot be permitted to identify a defendant in court if the defendant was initially identified as the result of an illegal arrest.

The dissenting judges in the 7-to-2 decision said the majority decision "permanently silences" a victim of crime whose testimony is unrelated to unconstitutional acts by the police."

Government prosecutors said yesterday that the appeals court decision could threaten future criminal cases in which the government's case is primarily based on identification testimony.

If defense attorneys can succeed in convincing the trial judge that their client was arrested illegally "we will wind up with no identification evidence," one prosecutor said.

Courts frequently exclude from trials physical evidence that was obtained in the course of an illegal arrest, such as narcotics. It is much less likely that identification testimony - usually based on memory - would be the product of an illegal arrest, attorneys said yesterday.

In an opinion for the majority, Judge John Ferren wrote that the Constitution "mandates our disapproval of the official misconduct which was designed to lead - and did lead - to the identification evidence in this case."

The decision reversed the 1974 armed robbery conviction of Keith Crews, then 16-years-old, in connection with an assault on a woman in a restroom in the vicinity of the Washington Monument on Jan. 3, 1974.

Crews was stopped at the monument several days after that incident by U.S. Park Police officers who along with a tour guide said Crews resembled the description of a youth seen in the area at the time of the assault.

When bad weather prevented the police from photographing Crews at the scene, he was arrested, taken to park police headquarters, photographed and released. The photograph subsequently was shown to the victim, who identified Crews as her assailant. She also identified him later at a police lineup.

At Crews' trial, D.C. Superior Court Judge Robert H. Campbell said the police had no probable cause to arrest Crews for any crime and refused to allow evidence based on the police photograph or the lineup. Campbell did allow the victim to identify Crews in court, in effect because the witness herself was an "independent source" for the identification.

The appeals court said, however, that suppression of the photographs and lineup identification was not enough to effectively deter official misconduct by the police.

Unless all the identification evidence was suppressed, Ferren wrote for the court, the police would be "prohibited from enjoying some, but not all, of the products of their wrong."