Two of the experts who planned the Postal Service's nationwide computer operations and three officials of its computer contractors have been indicted on fraud, bribery and racketeering charges involving almost $2 billion in potential government contracts.

The 58-count indictment, returned by a federal grand jury here, also charges that the scheme involved computer purchases for the Small Business Administration.

John L. Gentile, an assistant postmaster general from 1975 to 1980, is named as an unindicted co-conspirator. The indictment says Gentile helped devise the bribery scheme which started in 1977 and continued through 1983.

U.S. Attorney Joseph E. diGenova, who announced the indictment after it was unsealed yesterday, called the case a "very large and very lengthy conspiracy" that involved "behind the scenes information sharing," rigged bids and kickbacks.

"It's a classic example of government fraud and abuse," diGenova declared.

The two key figures in the alleged scheme are Ronald J. Perholtz, former general manager of the Postal Service's accounting division, and Franklin W. Jackson, former head of its office of financial planning and systems.

Perholtz, 37, was arrested early yesterday morning at his home in Silver Spring. He was released by U.S. Magistrate Patrick J. Attridge after agreeing to forfeit $500,000 if he fails to appear for arraignment Aug. 12.

The indictment says Perholtz, a certified public accountant, received at least $870,000 plus a 38-foot boat in kickbacks and fraudulant consulting fees as part of the conspiracy.

Besides Jackson, 38, of Fairfax, the others indicted were Gregory W. Fletcher, 39, of Arlington, former vice president of operations for International Business Services, of Washington; Lloyd E. Root Jr., 40, of Silver Spring, president of Computer Systems Support Corp., of Gaithersburg; and Conley H. Dillon Jr., 38, of Gaithersburg, a business associate of Perholtz and Jackson.

The indictment alleges Perholtz did extensive consulting work for several computer contractors, including Systems Development Corp., after he left the Postal Service in late 1977. It says Jackson moved back and forth between the Postal Service and private consulting. He resigned from the Postal Service two years ago.

Postal Service computer projects involved in the indictment are:

* ATAP, its automated time and attendance reporting system.

* STARS, an effort to link all timekeeping functions in a nationwide computer network, which was canceled last year.

* PRISM, or Postal Retail Information Systems Management, still under development, which is designed to automate post office windows by keeping track of transactions and customer records. After the indictment was announced, a Postal Service spokesman said the project was "under review."

* FIRM, a program to integrate major financial and accounting records.

At the Small Business Administration the indictment involves a nationwide system that tracks loans and collections.

Officials estimated that so far the two agencies have spent about $20 million on the projects.

The indictment says that Perholtz, Jackson, and Gentile concocted the bribery scheme in 1977. Perholtz then left the Postal Service, it says, to obtain consulting and royalty agreements with computer firms seeking to do business with the agency, although it says some of the firms, including Systems Development, did not know of any corruption involved.

But the indictment alleges that Jackson and Perholtz wrote specifications to favor the firms with which Perholtz had agreements.

The indictment said Dillon received $176,000 plus ownership of corporate property worth $144,000 and a $45,000 boat; Root received $143,000; Fletcher got $40,000 and a partnership interest in a Miami condominium, and Jackson received $27,000 and a partnership interest in the condominium.

If they are convicted of all the charges against them, Perholtz and Jackson could receive terms of 320 years in prison and fines of $123,000.