Making a quick start after his inaugural Friday, Mayor Harold Washington today named a civil rights lawyer the city's top legal officer and reached into Chicago's business community and the administration of former mayor Jane M. Byrne for other key appointments.

James D. Montgomery, 50, a trial lawyer who assisted the defense after the 1969 police raid in which two Black Panthers were killed, was chosen corporation counsel and will handle all civil legal matters for the new administration.

Washington named a veteran city fiscal officer, David F. Schulz, budget director. Schulz, now deputy commissioner of public works, will be in charge of crucial money-saving strategies.

The Washington administration says it believes the city deficit may be more than $400 million this year.

Washington said at his first City Hall press conference this morning that Chicago's finances are so grim that he may reassess his opposition to a real estate tax increase.

The new mayor, who outlined initial economy measures Friday, also said he will turn back $12,000 of his $60,000-a-year salary. He described the move as a gesture of austerity that other senior city officials are welcome, but not required, to emulate.

A senior fiscal expert in the Byrne years, Walter Knorr, was named acting comptroller.

A prominent North Shore businessman, Angelo Geocaris, was named counsel to the mayor, a $1-a-year post, which chiefly deals with business matters. Geocaris is chairman of Delphi Industries.

The mayor also named William F. Ware, 35, his chief of staff. His congressional office in Washington had been headed by Ware.

He appointed Sharon Gilliam, a former District of Columbia budget official, deputy budget director.

He chose Grayson Mitchell, 33, a former Washington Post reporter, to be his press secretary.

Throughout his first day as mayor, Washington received a steady stream of aldermen for brief private conversations. A crucial test of the reform-minded Washington's strength will come Monday when the City Council reorganizes itself.

Washington advisers have said that the mayor wants to oust County Democratic Chairman Edward Vrdolyak from his powerful post as chairman of the City Council's Committee on Building and Zoning.

While soft-pedaling the prospect of a bruising showdown Monday, Washington said he hoped "my reform propensities would permeate any discussions" opposing aldermen may be having with each other over the weekend.

Among the visitors was Alderman Vito Marzullo, the octogenarian dean of the council who bitterly opposed Washington during the primary and general election.

Marzullo, in a tailored three-piece suit and spring fedora, said that he "had no differences at all" with Washington in their chat today. He added that Washington's vow to fire hundreds of city employes hired recently by Byrne does not concern him. "I didn't get one of the 500, so why worry about it?"