On Monday a vociferous conservative will hand over power to a certified liberal in this city, the fourth-largest in the United States. The tone of city politics and tilt of policy both will change.

The retiring mayor is Frank L. Rizzo, the beefy former police commissioner who, in his eight years in city hall, became a hero in white, working-class wards, a symbol of repression among blacks and a comic figure for much of the world outside.

Rizzo served two terms. The voters refused to amend the city charter so he could seek a third.

His successor is William J. Green, son of Philadelphia's longtime Democractic boss, a wealthy former congressman and the successful candidate of the city's liberal reformists.

Green already has indicated that we will quickly reverse Rizzo policy on two highly sensitive issues -- the relationship between blacks and the police department, and neighborhood housing desegregation.

Charges of police brutality toward blacks were common in the Rizzo years. Last year the U.S. government even went to court against the Rizzo administration on grounds it had perpetuated and condoned police abuse, much of it targeted at blacks and other minorities.

A U.S. district judge threw out the charge. The Justice Department is appealing.

Green, meanwhile has promised to appoint a black to one of the three top jobs in the police department. That promise was intended both as an oblique warning to whites on the police force and as a reassurance to black citizens.

To some extent it has already worked. Green was supported in the election by the Guardian Civil League, a group of black police officers who often clashed with Rizzo. But the predominantly white Fraternal Order of Police, to which Rizzo still belongs, also endorsed Green.

As to housing, the Department of Housing and Urban Development had held up 90 percent of a $67.7 million comunity development block grant to Philadelphia because of the city's failure to provide enough low-income housing and its refusal to put such housing, likely to be occupied by blacks, in white neighborhoods.

HUD released $20 million of the money in October after the Rizzo administration complied with one of its demands.

It has indicated it will release another $20 million when ground is broken for the proposed Whitman Park housing project, 120 low-rent town houses to be built in a white section of south Philadelphia.

Rizzo promised the neighboring whites the housing would not be built during his time in office. Project propenents took him into federal court, however, and a U.S. district judge, saying Rizzo's opposition was racially motivated, ordered the project built.

Green has pledged to "obey the law" on Whitman Park. Construction is expected to start early in his administration.

In patching up Philadelphia's relationship with the federal departments, Green has been helped by the election calendar.Both President Carter and Sen. Edward M. Kennedy (D-Mass.) have sought his support in their race for the 1980 Democratic presidential nomination.

Kennedy, a longtime friend, campaigned for Green. Carter had planned to visit here one week after Green's victory, but his trip was called off because of his preoccupation with Iran. Green did, however, meet with Carter, Vice President Mondale and some HUD officials shortly after his election.

Federal money is especially important to Green because of the state of the city's finances.

Philadelphia faces a $7.6 million deficit, and Green probably will be forced early in his term to increase taxes and to cut back services, or at least city employment.

That will not please some of his supporters.

The president of the city's largest municipal union, which backed Green during the campaign, already has threatened to stage a one-day strike on inauguration day because Green did not consult him on appointments.

But "the city is fiscally sick," Green said recently. "In my judgment, stern meausres will be inorder . . . it needs a doctor who is prepared to administer the proper medicine." So the new mayor may be a penny-pinching liberal, a new kind.