aced with diminishing financial support from federal, state and local governments, philanthropic organizations and the private business community here have made a major commitment to two large urban redevelopment projects designed to halt the steady deterioration of the downtown area and to reverse the migration of small businesses to the surrounding suburbs.

The major undertaking in this effort is a $1.2 billion complex called California Plaza. Developed by a consortium of major business firms in cooperation with city planners, the project by decade's end will transform 11 acres of inner-city land formerly occupied by blighted Victorian homes into a mini-community of office buildings, condominiums, theaters and retail shops. The developers also are donating a $20 million art museum to the city.

California Plaza is being called the largest multi-use development of its kind in the nation. While some denounce it as adding to the city's already perilous downtown traffic woes, Mayor Tom Bradley and other city officials view it as a means to attract the young professional class back to the central city.

The project will be financed totally with private funds, said Martin Seaton, head of California operations for Cadillac-Fairview Inc., a Canadian firm that is a major partner in the venture. The land will be leased from the city redevelopment office under a long-term agreement.

"The land is owned by the government, and they will lease it rather than sell it to us over 99 years," said Seaton. "That's the way they wanted to do it. Other than that, the role played by government is a regulatory one." He estimated more than half a billion dollars in rents will be collected from the plaza during the next 30 years.

Financing the first stage of the project, scheduled to open in 1985, will begin in the next few months, Seaton said. "We require permanent financing for the first phase, and we believe it will be available, providing we're prepared to share in the ownership of the project. And we're prepared to do that."

City officials view California Plaza as the final link in a 15-year effort to revitalize the downtown area. "It will be a hub, an in-town place to live," said Robert Gilley, an executive vice president for Arthur Erickson and Associates, the project architect. "It will draw the suburban people back into town."

The complex, located near the intersection of three major California freeways, includes parking for several thousand cars, three 40- to 60-story high-rise office buildings, a 450-room hotel, 750 condominiums and dozens of retail shops and restaurants. The hope is that executives working in the office complex will be motivated to live in the area, adding stability to the population and reducing crime.

The second major project attracting considerable attention in the private sector here is a comprehensive central city skid row rehabilitation center that will offer a variety of services for the transient population. Called the Weingart Center after its primary benefactor, the late hotel magnate, the program is drawing wide support from the downtown business community, which views it as a way to recapture a 50-block area that has fallen into serious decline.

The center is being touted as a cost-effective alternative to jail and hospitalization for chronic street alcoholics. It will house a model treatment center for public inebriates, a medical clinic, job referral programs, a detoxification service and emergency housing.

The project has gained the support of Los Angeles' three major business organizations--the Chamber of Commerce, Merchants and Manufacturers Association and Central Cities Association--as well as the mayor and a host of public officials. Organizers say it will become a national model that can be replicated in other large cities where officials are attempting to reduce a large transient and inebriate population.

It was developed after the donation of a $2.9 million hotel by the Weingart Foundation, a $1 million grant for renovation by the foundation, a $1 million contribution from the Volunteers of America and a $3.4 million three year loan from a local bank. Nearly $1.9 million in federal HUD moneys administered by the city of Los Angeles also were used in the program's early stages.

Now a major $4 million capital fundraising drive is about to be launched using a special $25,000 grant from the ARCO Foundation. Aimed at local businesses and corporations, the campaign will emphasize the cost effectiveness of the center. Organizers say it will cost $35 a day to treat an inebriate at the center, compared to $171 to place a person in jail or $500 daily for hospitalization.

"Everyone including our governor is saying look toward the private sector," said Ed Eisenstat, director of the Volunteers of America Alcoholism Awareness Division. "That's exactly where we're looking. There has been an awful lot of support from the central business district."

Timothy Flynn, a Los Angeles attorney who directs the fund drive added, "The amount of interest demonstrated so far is very, very high. All three business groups have endorsed officially the Weingart Center, have endorsed the grant applications that were submitted on its behalf and have endorsed the capital campaign. There's a willingness to go to bat for its success."