Federal regulators seized control of a failing $3-billion-asset California savings and loan yesterday--the first time they have taken such action--after nervous depositors had withdrawn $70 million in one week.

Fidelity Financial Corp. of Oakland, the parent company of Fidelity Savings and Loan Association, was declared insolvent and a new federal mutual S&L was created, capitalized by promissory notes issued by the Federal Savings and Loan Insurance Corp. The government insurer will put in up to $100 million in notes. A directorate of former public officials and private executives was appointed, but the daily operations will be managed by Home Federal Savings and Loan of San Diego, and there will be no interruption of service at its 80 offices.

This is the first time the government has had to assume ownership of a savings and loan. Fidelity's size--it is one of the largest S&Ls in the country with $1.4 billion in deposits--made it too expensive to liquidate and too difficult to merge.

Although one official referred to it as a "nationalization," the government has no intention of remaining in the savings and loan business. The action was taken to buy time for the government to negotiate the sale of the S&L. A bidders conference will take place today in San Francisco, and a consortium of California S&Ls reportedly is forming to thwart out-of-state bidders. If a partner is not found, the S&L may be split among several institutions.

The Fidelity case set other precedents as well. The S&L filed a $114 million suit against the Federal Home Loan Bank of San Francisco, alleging that the bank had charged Fidelity too high an interest rate on its loans. Fidelity's board of directors refused to pass a resolution allowing regulators to merge it, and the S&L's officers refused to resign. But federal regulators acted to protect depositors and preserve public confidence in the banking system.

Fidelity Financial, with assets of $2.93 billion, suffered an operating loss of $56.9 million last year. Its troubles resulted from an aggressive lending policy in 1978-80, when it overcommitted itself to make fixed-rate mortgage loans at 11 percent to 12 percent. When interest rates failed to decline, as Fidelity's management had predicted, the S&L was obliged to borrow short-term at 18 percent to fulfill its loan commitments.

"Fidelity took excessive risks," said Jonathan Gray, an S&L analyst with Sanford C. Bernstein & Co. in New York. "Consequently it was squeezed to a much larger extent than the industry as a whole."

Its stock, which reached a high of $14 about two years ago, plunged to $2 before trading was interrupted last Monday. A.C. Meyer Jr., Fidelity's president, his former wife, and a San Francisco attorney owned about half of the 6.8 million shares outstanding. Stockholders will probably lose their entire investment.

For months rumors circulated of Fidelity Financial's plight and efforts at arranging a takeover within California and without. However, since the S&L was a state chartered association, approval by the California commissioner of savings and loans was required before a deal could be struck. The commissioner, Linda Tsao Yang, refused for some time to yield to federal regulators because Fidelity still had a $20 million net worth, or assets in excess of liabilities. (Federal regulations require merger proceedings if net worth slips below 2 percent; Fidelity's was under one percent.)

Then, at the beginning of April, Fidelity's auditors stated in its annual report that they doubted its continued existence. "That was the kiss of death," remarked Alan G. Bortel, an analyst with Shearson/American Express in San Francisco.

Yang recounted the sequence of events after that: During the week of April 5 depositors withdrew $70 million. (While an S&L can exist on little net worth, it must have sufficient cash to continue operations.) On April 9 the Federal Home Loan Bank of San Francisco informed Fidelity, which had already borrowed $1.3 billion from the bank, that it would advance no further funds unless they were guaranteed by the FSLIC. The federal insurer said it would not guarantee further advances unless it took possession of the S&L and appointed receivers.

On April 13 officials of state and federal agencies walked into the office of Fidelity's president and announced they were seizing control of the thrift's property, assets and liabilities. Under the circumstances, Yang said, no other solution was realistic.