Mayor Marion Barry assured officials of the city's labor unions last night that city employes will get their previously negotiated 7 percent pay raise set for later this year despite the District's financial problems.

"No negotiated settlements are in jeopardy," Barry was quoted as saying by one of the approximately 20 union leaders who attended a private, unannounced meeting with the mayor at the District Building. Moments before the session, which was not listed on the mayor's official schedule, began at 6:45 p.m., Barry described it as a "coffee and tea social with my labor friends."

But after the 90-minute session, also attended by Alphonse Hill, deputy mayor for financial management, Barry confirmed that the purpose of the meeting was to reassure the nervous labor officials that the 1984 fiscal year raise set for Oct. 1 is secure.

Last week, a high city government source said Barry privately had discussed ways to persuade the unions to forgo part of the raise to help balance the city's books by making the unions appear selfish if they didn't.

"We are convinced that the terms of the contracts will be carried out," said Joslyn Williams, president of the Metropolitan Washington Council, AFL-CIO, the area's umbrella labor group. "We take the mayor at his word."

Earlier yesterday, representatives of several city agencies, including the corporation counsel's office and the fire and corrections departments, appeared before the City Council to discuss their portions of Barry's $1.99 billion budget for next year.

Barry's projection that the city's prison population will decrease next year, despite near-record levels of incarceration now, emerged as a major issue during the day's hearing.

Chief Judge H. Carl Moultrie I, head of the D.C. Superior Court, disputed the projection that the inmate population will decrease substantially next year.

"I see no reduction at all, none whatsoever of a reduction," Moultrie said in response to a question from council member H.R. Crawford (D-Ward 7). "Let's be frank about it. There are some people who need to be in jail."

Moultrie was among several judicial officials yesterday who questioned estimates by Barry's budget officials that project a drop of about 300 inmates in the city's average of 5,500 inmates at Lorton prison and the D.C. Jail.

A representative of the city's public defender service said new legislation that allows monetary fines instead of prison sentences would limit the number of prisoners. But he said the mandatory minimum sentencing act, which takes effect March 1, would probably increase the number of persons in jail.

Bernice Just, chairman of the city's parole board, said there was little her board could do to speed up parole hearings. She said the board had not been consulted on ways to reduce the number of prisoners.

William Golightly, the chief budget official for the Department of Corrections, testified later in the day that the city hopes to reduce the inmate population by instituting limited sentencing and alternative sentencing for nonviolent prisoners.

Elizabeth Reveal, Barry's budget director, said the city, which has the highest incarceration rate in the country, expects to use "alternative sentencing" because "we've got to."

On other budget issues, Moultrie said Barry's proposed budget of $33 million for the court for the fiscal year that begins Oct. 1 is about $3 million less than he requested and failed to add about 35 new positions. "The court must increase its employes if it intends to be an instrument of justice in this city," Moultrie said.

Corporation Counsel Judith Rogers, defending a 1984 budget of $6.5 million that includes 12 additional positions, said the increase would only recoup losses expected this year.

She said if the council does not approve the additional positions, her attorneys may have to cut back on increased prosecution of cases involving drunk drivers, housing violations and child abuse.