Perpetual American Federal Savings and Loan will open brokerage offices at five of its local branches on Tuesday, plunging the savings and loan industry into a business that has been the nearly exclusive preserve of the securities industry for 50 years.

At Perpetual's new facilities, customers will be able to buy stocks, bonds, mutual funds and government securities as well as receive limited financial counseling.

The brokerage service, called Invest, was founded by Perpetual and three other large thrift institutions -- Coast Federal in Florida, City Federal in New Jersey and California Federal. By the end of the week Invest offices should be operating at 27 branches of the four founding savings and loan associations.

Invest officials said that by the middle of 1983, brokerage offices will open at another 24 S&Ls that bought smaller interests in the service and that hundreds of other associations have inquired about offering the Invest services.

The move by Perpetual and the others is an attempt by a battered industry to find new sources of income.

The industry prospered for more than 150 years mainly by gathering low-cost deposits from small investors and lending those deposits to home buyers. But in recent years, the costs of obtaining deposits skyrocketed far beyond the income S&Ls earned from the long-term mortgage assets. The industry has lost billions of dollars a year for the last few years.

By venturing into the brokerage field, the savings and loans in effect will be striking back at the industry that has caused them the most difficulties since 1978. Money market mutual funds, created and sold by brokers, enabled hundreds of thousands of smaller investors to obtain "market" rates for their funds that they could not get at tightly regulated banks and savings and loans.

The securities industry has fought the Invest move ever since the Federal Home Loan Bank Board, which regulates S&Ls, approved the venture last spring.

Bert Auer, marketing director at Perpetual, said Invest's commission charges will be somewhere between those offered by full service brokerage firms such as Merrill Lynch and by barebones discount brokers such as Quick & Reilly.

Auer said that Perpetual's market research has shown that there are a large number of individuals who have the financial resources and the desire to deal with a broker, but who would prefer to do it through a known quantity -- such as the savings and loan with which they have deposits and mortgages -- rather than go shopping for a brokerage firm.

Commercial banks, some of which are also trying to break into the securities business in one way or another, say their research shows a vast pool of untapped potential brokerage customers, although some bankers and analysts are skeptical that there are many investors in the country with a need for a broker who have not found one.

The entrance of the savings and loan industry into the securities business is but the latest in a series of moves that has blurred the once rigid demarcations in the so-called financial services industry.

Prudential Insurance Co., the country's biggest insurer, bought The Bache Group Inc., one of the country biggest brokerage firms. Brokerage firms like Merrill Lynch & Co. have moved into real estate sales, insurance sales and with the advent of so-called cash management accounts have acted as banker to a number of their customers.

Sears, Roebuck & Co., the country's No. 1 retailer, has purchased the major brokerage house Dean Witter & Co. and the big real estate firm Coldwell Banker. Several months ago Sears, as an experiment, opened offices at eight stores across the country that offer real estate and brokerage services using Dean Witter and Coldwell Banker personnel. One such Sears center opened at suburban Fair Oaks Mall in Fairfax County.

Several banks have begun to offer their customers brokerage services, either by contracting with a going concern or trying to buy one.

Perpetual's office at 11th and E streets NW will be the site Tuesday of the nationwide kickoff for Invest. Perpetual branches in Silver Spring, Bethesda, Alexandria and McLean also will open brokerage offices Tuesday.

Auer said there will be two registered representatives -- brokers -- in each office who will be salaried, rather than work on commissions. One broker in each office has been hired from a securities firm while the other is a long-term Perpetual employe.

The Invest outlets will be linked to the company's central facility in Tampa. The actual transactions for Invest customers will be carried out by Pershing & Co., a division of the big brokerage firm Donaldson, Lufkin & Jenrette.