BOSTON – Bank of America Corp. completed its $47 billion acquisition of FleetBoston Financial Corp. on Thursday, creating the nation’s No. 3 bank with assets estimated at $966 billion.

The bank has about 35 million consumer and small-business customers, about 5,700 branches from coast to coast and 16,500 ATMs. Its assets trail only Citigroup and the planned merger between Bank One and J.P. Morgan Chase.

Fleet spokesman James Mahoney declined to comment on how many layoffs would be coming as a result of the merger. Some estimates have put the figure as high as 13,000, or about 7 percent of the banks’ combined work force of 181,000.

“The process is under way to finalize the number of job reductions and we will be providing more information when that process is complete,” he said.

For consumers, the only immediate difference as a result of the merger is that Fleet and Bank of America customers are now able to use the other’s ATMs across the nation without charge, said Fleet spokesman Joe Goode.

“In the near term, customers’ experience day to day will remain the same,” Goode said.

Fleet customers will begin to see signs and marketing materials change at their banks as soon as July, with all the changes completed by the start of 2005, he said.

About 10,000 Fleet employees across the Northeast were to participate in internal meetings Thursday to discuss the merger, said Goode.

Fitch Ratings lowered Bank of America’s credit ratings Thursday because of the “integration issues inherent in a merger.” It raised FleetBoston’s ratings because the merged company “is expected to be a significantly stronger organization than [Fleet] had been previously.”

Shares in Charlotte, N.C.-based Bank of America rose 63 cents to $81.61 in afternoon trading on the New York Stock Exchange.

Kenneth D. Lewis, chief executive of Bank of America, will hold the same position at the combined bank, which retains that name. FleetBoston CEO Chad Gifford becomes chairman of the new bank.