Frederick D. Dorsey, Mayor Marion Barry's nominee to replace People's Counsel Brian Lederer, said yesterday he intends to continue Lederer's practice of aggressively advocating the public interest at utility rate hearings.

"I can appreciate any concern that ratepayers might have that somehow this change reflects on the vigor of protecting consumer interests," said Dorsey, the city's principal deputy corporation counsel since 1982. "I can assure you. . . there will be no dimunition of the vigorous protection of consumer interests."

Dorsey's comments came after some City Council members and spokesmen for consumer groups expressed dismay at Barry's announcement Wednesday that he will replace Lederer, who had wide support from consumer groups in his six years in the job.

Consumer spokesmen expressed concern that the decision might signal an attempt by Barry to accommodate the interests of utility companies, who have sharply criticized Lederer. The people's counsel represents the public in utility hearings before the Public Service Commission (PSC), which sets rates for gas, electric and telephone use.

Dorsey, who has little direct experience in utility cases, said he is not yet ready to take a position on a City Council bill that would block the PSC from rejecting the expenditures the people's counsel incurs in rate cases--legislation that Lederer has said is essential to combatting the well-financed cases presented by utilities.

Utilities, which by law are required to pay the costs of the people's counsel cases, have charged that Lederer's litigation costs are excessive. The PSC did not claim the authority to control people's counsel expenditures until after Lederer took office.

Lederer has charged that the utilities are using the issue to diminish his effectiveness. When he took over in 1977, Lederer transformed the scope of the office, largely by hiring outside experts, including economists, engineers and lawyers, to scrutinze the complicated data presented by utilities.

"Brian was highly regarded for his expertise, especially in natural gas cases and in telecommunications areas," said Fred Goldberg, the lawyer for the National Association of State Utility Consumer Advocates.

"I think it's completely inappropriate to appoint someone not versed in utility matters to replace him," said Goldberg, adding that such expertise will be especially important because of the major rate changes and other issues that are expected to result from the court-ordered breakup of the American Telephone & Telegraph Co.

For example, as part of an $82 million rate increase already pending before the PSC, the the Chesapeake & Potomac Telephone Co. is asking for approval to charge for some local calls as if they were long distance, the so-called "optional measure service." C&P has argued that the prosposal would be a money-saving alternative for persons who rarely use their phones. Lederer successfully argued for the defeat of a similar proposal last year.

Dorsey, 42, was an associate general counsel for class action cases and director of policy for the U.S. Equal Employment Opportunity Commission and was an assistant and then acting general counsel for the U.S. Commission on Civil Rights before joining the corporation counsel. As people's counsel he would receive $56,301 a year, his present salary.

Lederer, 38, was nominated for his first three-year term in 1977 by Mayor Walter E. Washington and was reappointed for a second term in 1980 upon Barry's recommendation.