SAN FRANCISCO (AP) _ Shareholders of BankAmerica Corp. on Thursday approved a merger with rival Security Pacific Corp., a combination that would produce the nation's second largest bank and could cost thousands of jobs.

Bank officials maintain the move will ultimately save $1.2 billion a year, increase efficiency and pump up the bank's financial muscle.

''This is the right move, at the right time, for the right reasons,'' said Chairman of the Board Richard M. Rosenberg, who estimated the deal will be closed by spring 1992.

But some worry that the combined bank, which will take the name BankAmerica, will monopolize markets and make loans harder to get. Layoffs are also a concern, with bank officials saying they don't know how many jobs will be lost. Analysts estimate thousands will be cut from the payroll.

''I think big bank mergers are destabilizing to the whole banking system,'' said Tom Schlesinger, executive director of the independent research group, Southern Finance Project in Charlotte, N.C. ''They're obviously going to cost thousands of jobs. They're going to exacerbate already troubled real estate markets by putting more vacant commercial real estate atop an already overbuilt market ... there's kind of a triple whammy.''

But bank officials say a recession makes cost reductions more important than ever.

''There are just too many banks in this country chasing too few customers,'' Rosenberg told shareholders.

He declined to put a figure on expected layoffs, although he said 60 percent of the savings are expected to come from personnel. He said hiring is under stringent restrictions to leave as many positions open as possible.

Rosenberg said the company's severance provisions allow three weeks pay for each year of service up to a maximum of 18 months. The company also will offer job placement and retraining programs, he said.

The merger was approved by 98 percent of the votes cast by BankAmerica common stockholders. In Los Angeles, 97.75 percent of Security Pacific common stockholders approved the combination.

The new banking company would be a 10-state network with $190 billion in assets, second only to New York-based Citicorp's $217 billion in assets. Rosenberg would be the new chairman and Security Pacific Chairman Robert H. Smith would be president of the new bank.

The merger still must be approved by federal banking regulators and the U.S. Justice Department. Rosenberg said early indications seemed positive.

Analyst Joe Arsenio of Hambrecht & Quist in San Francisco said concerns about the merger tend to be short-term.

While the real estate market could soften, creating the need for more bad loan reserves, ''none of that is new. Reserves have been taken, provisions made and adjustments proposed,'' he said.

Concerns raised at Thursday's meeting included the advisability of taking on Security Pacific because of its troubled loan portfolio and whether consolidating branches will squeeze credit availability in poor neighborhoods.

Security Pacific recorded a net loss of $509 million in the third quarter of this year, reflecting a $1.2 billion provision for bad loans. Rosenberg said fixing Security Pacific's loan portfolio was a priority. But he noted that Security Pacific's problems were one reason BofA was able to get a favorable 0.88-for-one stock swap ratio.

Shareholder Robin Cannon said she worried that the merger would take banks - and loans - away from her low-income Los Angeles neighborhood.

''I think we'll see more disenfranchisement of my community,'' said Cannon, a member of a group called the Coalition for Accountable Reinvestment.

Rosenberg said BofA will not abandon communities that don't have enough banks. And he said the bank has a $12 billion goal of lower-income lending in the 10 years following the merger.

''It's a nice number and a nice sentiment but the proof will be in the pudding,'' said Schlesinger, who maintains that fewer, bigger banks will drive up the price of credit for all borrowers.