Major banks around the nation reduced their prime interest rates one-half point yesterday, more than two weeks after the Federal Reserve encouraged such a move to stimulate the economy.

The big New York banks, led by Citibank and Morgan Guaranty Trust Co., were the first to drop their benchmark rates yesterday morning, and they were followed by at least 15 other banks later in the day. Regionally, C&S/Sovran Corp. and Crestar Financial Corp. were among those cutting their rates from 10 percent to 9.5 percent. California-based BankAmerica Corp. became the first major bank to drop its prime Monday.

Analysts said the drop in the prime had symbolic value at a time when consumer confidence is very low. But, many said, the decrease likely was not enough to turn around an economy widely believed to be in recession.

"My guess is it will take a while for consumers and businesses to step forward and take advantage of the lower rates," said Alan Gayle, chief economist at Crestar. "When confidence is low, a drop in the prime does not necessarily bump up demand for loans. It's a little like pushing on a rope."

Ivan A. Marcotte, financial economist for the North Carolina bank company NCNB Corp., which also cut its prime, added: "I think it is too little and too late."

The prime rate, to which many consumer and business loans are pegged, stayed at 10 percent for all of 1990. Toward the end of the year, as various measures of the economy's performance began sinking quickly, the Fed, the nation's central bank, began making it easier for banks to drop their lending rates.

On Dec. 18, the Fed reduced its discount rate, a key rate it charges to banks when they borrow directly from it, from 7 percent to 6.5 percent. Often when the discount rate changes, money-center banks respond quickly. This time, with the banking industry in a weakened financial condition, banks appeared to hold on as long as they could before competitive pressures forced them to follow.

The decrease is likely to push down the interest rates individuals pay on their consumer, auto and home loans -- but not by much, and perhaps not soon, experts said. Credit card rates are notoriously immune to market conditions, and home mortgages, whose terms are much longer than those for loans made at the prime, are set with an eye to future inflation as well as the cost of funds to lenders.

Home mortgages have fallen so much lately they may get little extra push from the lower prime rate.

"My own view is that mortgage rates have not been the problem in housing. Accordingly, a decline in mortgage rates won't provide all that much lift," said Donald Straszheim, chief economist for Merrill Lynch & Co. in New York. "We have simply got an economy that has runout of gas, pushed over the edge by a big rise in oil prices associated with the Persian Gulf."

Some consumers, however, will benefit directly from the decline in the prime since many home-equity loans are linked to the benchmark rate and could be expected to go down when they are next adjusted. Marcotte of NCNB said the decline would be about $2.50 a month for a loan of $10,000.

Falling rates also could induce more homeowners to refinance their mortgages to reduce monthly payments. While that might not be the best thing for the lenders holding the mortgages, refinancings would be good for the economy because they would put more money in consumers' hands.

The falling prime rate caused a rally in the bond markets; declines in interest rates typically cause bond prices to rise. However, the stock market responded to the interest rate decline unenthusiastically, with other factors pushing the Dow Jones industrial average down 23.02 points, to 2,610.64.

In a sign of market concern about bank profitability, some bank stocks fell a bit after the prime-rate declines; others rose. small amounts. Citibank was up 12 1/2 cents to $12.75, for example, while Morgan Guaranty was down $1.25 to $43.25 and C&S/Sovran was unchanged.