WASHINGTON (AP) _ Passage of a bill allowing legal wiretaps of cellular phone conversations removes an important obstacle to effective law enforcement, FBI Director Louis Freeh said Saturday

But privacy advocates said the bill, expected to be signed shortly by President Clinton, represents an erosion of individual freedom.

''The FBI had been thwarted in important cases by sophisticated digital technologies already installed on some telephone networks,'' Freeh said in a statement after the Senate passed the bill Friday.

''Without this bill ... we would have been completely prevented in a very short time from carrying out any court-approved wiretapping,'' Freeh said.

Defeat of the bill would have struck at the heart of the FBI's ability to use court-ordered wiretaps to probe drug trafficking, organized crime, terrorism and kidnapping, Freeh said.

But some critics said the FBI had not proved that the bill provided an essential crime-fighting tool.

Marc Rotenberg, project director of the Electronic Privacy Information Center, a non-profit group, said the organization would pursue a lawsuit ''to obtain information from the FBI director that he claims justifies this legislation.''

Rotenberg called the Senate action a disappointment, and added: ''We believe it is a matter of right that individuals are entitled to privacy in their communications and we do not believe the FBI made a sufficient case to justify this legislation.''

Dean St. Dennis, an FBI spokesman, disagreed, saying, ''The FBI carefully documented the problem it was facing and worked with Congress to make a great deal of information available.''

Rep. Don Edwards, D-Calif., chief sponsor of the legislation in the House, said during its consideration that the FBI had documented 183 instances in which wiretaps were frustrated by cellular phones, call forwarding, voice mail, call waiting or other new phone services.

Edwards said that while some might object to the bill, wiretapping is permitted under U.S. law as a legitimate investigative tool.

''And law enforcement agencies are entitled to use it,'' he said.

The bill, passed by the House on Wednesday, would authorize spending $500 million over three years for equipment to be installed in telephone and cellular networks to permit legal wiretaps.

It would require phone companies to make the changes in their networks and services that will ensure that court-ordered wiretaps are effective.

Once investigators pinpoint a target telephone number they would have to be able to track and record calls made to and from it.