Banks, savings and loans and consumer groups yesterday declared their unanimous opposition to the proposed acquisition of the ailing Fidelity Savings and Loan of San Francisco by Citicorp of New York.

The takeover, which already has been approved by the Federal Home Loan Bank Board and is now before the Federal Reserve Board, represents the first merger in many years of a savings and loan and a bank holding company across state lines. Approval would profoundly affect the U.S. banking system.

At the first of two days of hearings by the Fed, witnesses used legal, economic and even moral arguments against the merger between the country's second-largest bank holding company and the California thrift institution.

Permitting Citibank to expand its California operations in this way would have broad, adverse consequences, they testified, including a de facto repeal of laws prohibiting interstate branching and the draining of capital from the state for investment overseas. A decision in Citibank's favor also would set a precedent for other banks to take over thrifts, leading to the disappearance of the industry, other witnesses said.

The U.S. League of Savings Associations claimed Citicorp would present unfair competition for smaller California financial institutions. The Independent Bankers Association charged Citicorp with trying to set up de facto retail banking outlets nationwide.

All speakers complained that a very important issue was being decided with undue haste by administrative agencies and urged that it be left up to Congress, which is now considering banking legislation. They argued that the bill sets merger priorities that might well have meant awarding Fidelity to a California institution.

Citicorp's attorney said the New York giant would keep Fidelity in the mortgage loan business, that its competition would benefit consumers and that it would not siphon funds from the thrift.

The bank board said that it approved the acquisition because Citicorp submitted an offer $143 million better than the closest bidder. Attorney Ernest Leff, representing Fidelity Federal Savings and Loan of Glendale, Calif., yesterday disputed that contention, claiming that Glendale's overall bid was better than Citicorp's.

Last month bank board Chairman Richard Pratt wrote Fed Chairman Paul Volcker, saying that, under the most favorable interest rate assumptions, the Federal Savings and Loan Insurance Corp. would have to give a subsidy of $33 million to Citicorp, or $56 million less than it would have to give Glendale. Leff countered that the subsidy would be $41 million for Citicorp and $20 for Glendale. The bank board refuses to release financial details of the bidding.

From the start, the Fidelity case has been controversial. The troubled state-chartered S&L was seized last April when its net worth dropped to $20 million, and it was transformed into a federally insured S&L over the objections of California S&L Commissioner Linda Tsao Yang. A suit challenging the legality of the seizure was decided in favor of Fidelity, but overturned last week on appeal. The bank board then solicited bids for Fidelity and indicated Citicorp was the winner. At the insistence of California banks and savings and loans, its congressional representatives and commissioner Yang, a second round was held with the same result.

Yang continued to object to the bank board's methods until last week, when Gov. Jerry Brown's secretary of business, transportation and housing suspended her for "insubordination" and forbade her to talk to the press about Fidelity.