Attorney General-designate Griffin B. Bell said yesterday that FBI Director Clarence M. Kelley will be replaced "before too long" as head of the federal government's principla law enforcement agency.

Testifying at the Senate Judiciary Committee's hearings on his confirmation, Bell replied to a question about Kelley by saying: "I think we can look forward to the time when we'll have a new director."

Bell said he couldn't give the committee a "time frame" for the FBI changeover. But, he added, "I think it would not be long."

In his remarks, which came during the committee's morning session, Bell was reluctant to discuss details of the incoming Carter administration's plans for the FBI. And, when he returned to testify in the afternoon, he seemed to hedge slightly about Kelley's status.

When committee members pressed him to be more specific, Bell said that during the luncheon recess he had received a number of calls from Kellys supporters in Congress.

"I'd like to stand on what I said this morning," he said.

In his earlier testimony, Bell had praised Kelley for his three-year effort in trying to restructure the scandal-torn FBI and repair the damage caused to the bureau's reputation by its past abuses of civil liberties in domestic security investigations.

But he also seemed to confirm in an indirect wasy the widely held belief that the Carter team feels Kelley has not succeeded in exerting control over the FBI and wants a director of its own choosing.

Bell noted that, under a new retirement law, most of the FBI's top management echelon will be leaving at the end of the year and that it is likely that Kelley will stay on only for a time to assist with the transition to a new executive team.

He pointed out that Kelley, 65, was appointed to a term that would normally enable him to remain in office until at least age 70. The "legislative history" of how the law would apply in case of an attempt to drop Kelley earlier "is not clear," Bell said.

However, congressional sources familiar with the legislation granting the FBI director a single term of 10 years said it was not intended to bar a President from replacing him before the end of that term. The director, these sources said, still "serves at the pleasure of the President" and can be removed by him.

Although Bell did not mention it, another factor known to be complicating Kelley's situation is his pension. Under a federal law passed last summer, the three years that Kelley has served as director would allow his pension from earlier FBI service to be recomputed on the basis of the higher director's salary.

However, the law does not take effect unitl Oct. 1, and Kelley would have to remain in active federal service would that date to take advantage of its provisions. For the reason, Bell is believed to be sumpathetic to keeping Kelley at the FBI in some capacity until October.

Bell also reiterated that he ahs not decided on a new FBI, director. But he told the committee that, in interviewing candidates for the still unfilled post of deputy attorney general, he has been weighing how they would fit into the FBI director's slot.

Although Bell's remarks about Kelley created the biggest stir at yesterday's hearing, most of the questioning by committee members continued to focus on his civil rights positions during his earlier careers as an Atlanta lawyer and a federal appeals court judge.

His nomination to head the Justice Department has stirred heavy opposition from a number of civil rights and liberal groups, and representatives of these organizations are scheduled to begin testifying against Bell's confirmation today.

Much of the opposition centeres on charges that during the late 1950s he helped former Georgia Gov. Ernest Vandiver's program of "massive resistance" to school desegregation.

Bell has insisted that he tried to be "a force for moderation" in his legal advice to Vandiver, and yesterday he summarized his role during that period by saying:

"I do not have any regrets about participating in the Vandiver administration. I think I made a valuable contribution to the times I was in. But that doesn't mean my views haven't changed. I hope I've grown some . . . I think I'm a person who believes in civil rights."

He admitted that he belongs to two Atlanta clubs - the Capital City Club and the Piedmont Driving Club - that bar blacks and Jews from membership. He has agreed to resign from both clubs, reportedly under the pressure from President-elect Jimmy Carter.

Bell said he also had been a member of a laywer's club that had barred blacks and that he had fought to have this restriction removed.

"Because of that issue, I did not attend a meeting for three years," he said, "I drew a distinction in my mind between a lawyer's club and a social club. One involves public services, and I do not believe it should be restrictive in its membership."