Consumer groups urged Congress yesterday to put a ceiling on credit card interest rates.

With interest rates on credit cards still averaging 18.6 percent, the consumer groups, along with several Democratic legislators, called for a nationwide, variable cap pegged to a federal index.

The legislation under consideration by a House Banking subcommittee would limit the maximum allowable charge to 5 or 6 percentage points above the Federal Reserve's discount rate or the 3-month Treasury bill rate. Under current rates, that would reduce interest charges on credit cards to between 12.5 and 13.5 percent.

Such a reduction would put them more in line with other consumer rates, which have fallen dramatically in the past four years.

The idea of a federal interest rate limit aroused strong opposition from the banking industry and the Federal Reserve, which warned that fewer people would be able to obtain credit and that the cost of goods and services could rise.

There are approximately 700 million credit cards in use, and revolving consumer debt stands at an all-time high. Bank card revenue alone totaled $8 billion last year. Only three states have credit card interest rate ceilings below 18 percent. Rates range from a variable 12.5 percent in Arkansas to a statutory 30 percent in New Jersey. Seventeen states set no ceilings.

Under a plan proposed by Rep. Charles E. Schumer (D-N.Y.), the savings on bank card interest charges would amount to $4 billion a year. According to Rep. Mario Biaggi (D-N.Y.), the "rip-off of the American consumer" could be reduced by about $2 billion a year in interest charges.

The change in market interest rates has caused a kind of Catch-22 situation in the credit card industry. As the competition has become keener among issuers entering this lucrative business, so has the need to enlist more customers. Instead of driving interest rates down, however, the industry's practice of sending out unsolicited, pre-authorized credit lines has resulted in more deadbeats, which, in turn, drives up the price of credit.

Federal Reserve member Martha R. Seger argued that a federally mandated ceiling on credit card interest rates could lead to difficulties just the way the imposition of credit controls in 1980 did. She reiterated the Fed's opposition to interfering with the free market.

Robert W. Johnson, director of the Credit Research Center at Purdue University, predicted that a rate ceiling would boost annual costs, eliminate the free period before charges are assessed and cause merchants to pay credit card companies more, thus increasing the price of the goods. His center is supported by grants from the credit industry.