The Saw Mill River Parkway slices like a jagged knife through this segregated New York City suburb, where low-income housing projects are relegated to the poorer west side and expensive, split-level homes cover the hills to the east.

When Herman Keith tried to move across the parkway to a white neighborhood, a real estate agent told him the owner would not sell to a black family. When he and other black families moved to a nicer west side area, he said, school boundaries were redrawn so their children remained in largely black classrooms.

Now Keith, the NAACP and the Justice Department are challenging these suburban traditions with an unprecedented lawsuit that charges the state's fourth-largest city with maintaining a deliberate pattern of housing and school discrimination.

The suit was filed in the final days of the Carter administration and is scheduled to go to trial July 5. The Reagan administration, which has filed only five housing bias suits and generally opposes school busing, briefly considered dropping the Yonkers case but decided to press ahead.

The bitter debate here resounds with echoes of Chicago's recent mayoral campaign. Michael Cipriani, a Republican councilman who has led the battle against low-income housing in his largely Italian ward, said he does not understand why the federal government is picking on Yonkers for something that is common in most cities.

"My home is worth $225,000," Cipriani said. "I don't want any apartment building next to me. That's what I moved to the suburbs for. What does the federal government want? Do they want white flight taking place in the city of Yonkers?"

But many black residents see the lawsuit as long overdue. Although blacks and Hispanics make up more than 18 percent of the population in this Westchester County community of 200,000, there has never been a black member of the city council, there are no black department heads and, until the Justice Department filed a second discrimination suit, there were only 13 black and Hispanic officers on a 487-member police force.

"I equate Yonkers with Biloxi, Mississippi, in 1950," said Keith, who is president of the Yonkers NAACP. "You look around and see an absence of blacks in every area."

Keith said his worst suspicions were confirmed in 1979, when he was laid off as senior program manager in the community development agency after 13 years. He filed a discrimination suit and won a $35,000 settlement.

The history of official segregation here can be traced back more than 40 years, according to court documents and attorneys involved in the case.

As thousands of New Yorkers fled to Yonkers, the more affluent white families bought one-acre homesites on the east side with mailing addresses in adjacent and exclusive Scarsdale. Blue-collar workers moved to cheaper east side apartments. Few blacks, however, ventured from the west side.

Over the years, the city's housing agency has suggested more than 140 east side locations for subsidized projects, and each one has been rejected by the city council.

Since the early 1970s, the Department of Housing and Urban Development repeatedly has threatened to cut off the city's federal aid unless it built some projects on the east side. But what the newspapers called "Negro housing projects" continued to fill the southwest ghetto.

Mayor Angelo Martinelli, a Republican, described this as "a natural tendency. America was built on the fact that you could buy a house that was near your school, near your church, and in an area you wanted to live in. The Italians settled on Park Hill Avenue, the Irish settled on Lake Avenue and the blacks settled in the western part of Yonkers."

Two years ago, however, HUD finally pressured city officials into submitting a list of 14 possible housing sites east of the Saw Mill River Parkway. The only site remaining on that list is School 4, an abandoned building in Cipriani's ward. Instead of following its usual practice of advertising for public bids from developers, the council allowed Cipriani to appoint a neighborhood committee to decide the school's fate. Emotions boiled over at a public hearing.

"It was a most vituperative outpouring of hatred," said Michael Sussman, an NAACP lawyer. "People got up and started screaming about how they had fled from the Bronx to get away from blacks. It was just a period of total racial hysteria."

Soon afterward, when one developer inquired about building low-income apartments, the neighborhood committee told him not to bother making a bid. A deal already had been made with another contractor to put up luxury condominiums and townhouses valued at more than $100,000 each. But Justice Department officials recently got an injunction to block the condominiums.

"We're going to derive lots of income from townhouses and condos," Cipriani said. "What the hell are we going to get from subsidized housing? We've got 50 percent of the total low-income housing in Westchester County . . . ."

Mayor Martinelli said the government is being hypocritical.

"Where was the Justice Department when HUD was giving us millions of dollars to build subsidized housing on the west side of Yonkers?" he asked. "The federal government said 'Hey, you want to build it over here in a slum area, fine.' Now they're coming back 30 years later and changing the rules in the middle of the game."

In any case, Martinelli argued, federal funding for low-income public housing has all but dried up.

Similar bitter arguments have swirled around the city's schools, which are more than 90 percent white on the east side and largely black to the west. The Justice Department suit said the city has maintained segregation by building certain schools and closing other ones, altering attendance zones and assigning teachers to schools on the basis of race.

"There's no question there is a decided, definite racial imbalance in the school system," said school Superintendent Joan Raymond. "Anyone with eyes can see that."

Raymond, a former school official in Chicago, acknowledged that the Yonkers school board has rejected two modest integration plans over the last five years because of community opposition to busing. But she said the Justice Department has refused to consider her recent compromise proposal, which would set up integrated learning centers with computer equipment and bus elementary students there twice a week. "The Justice Department has one approach and one approach only--statistical parity, the numbers game using children," Raymond said.

"The result is that in five, six or seven years, this will probably be a totally minority school system."

The divisive mood here could intensify in the coming weeks if Keith decides to challenge Martinelli and several other white candidates in a long-shot campaign for mayor.

"There's a possibility we could have a Chicago here," Keith said. "If I could muster 90 to 95 percent of the black vote and a portion of the whites and Hispanics, I might be able to pull off an upset.

"I am perceived by a lot of white folks as threatening their American dream," he said. "But I don't think white people should be threatened by us. We need to start talking about a united city."