The Federal Reserve Board, responding yesterday to increasing signs that the nation is heading into a severe recession, began dismantling the credit controls it imposed March 14.

In effect, the board cut in half the impact of the various special controls, including those applying to credit cards and other forms of consumer credit.

The White House called the Fed's moves "appropriate in view of the success in moderating credit demands and reversing inflationary psychology since the March 14 program was announced."

The White House said "the Federal Reserve actions will help to assure greater availability of funds for lending to small businesses, farmers, automobile dealers and buyers and many other borrowers."

In its statement, the White House also jawboned the nation's banks to cut interest rates charged on loans. "Since the cost incurred by banks in obtaining lendable funds will be lower, the rate that banks charge their customers should also decline. The administration is particularly concerned that the prime rate of interest has fallen much less than other interest rates in recent weeks," it said.

Chase Manhattan Bank, the nation's third largest, began charging its best customers 15 1/2 percent yesterday, down from 16 1/2 percent, the prime rate still being charged by most banks.The prime has now dropped from a peak of 20 percent, but some other rates have dropped much more.

The Fed moved at a time most economists are revising forecasts to show what likely would be the second-worst recession since World War II. Economic activity, adjusted for inflation, is expected to drop at near record rates this quarter and continue to fall for the rest of the year. Unemployment last month jumped from 6.2 percent to 7 percent, and most forecasters expect it to top 8 percent by fall.

The Federal Reserve cut from 15 percent to 7 1/2 percent the portion of any increase in many types of consumer credit that larger lenders must set aside in special non-interest-bearing accounts at the Fed.

At the same time, the Fed moved the base period used to calculate lenders' outstanding credit forward from March to June and said no deposits will be due until July 24.

Lloyd Hackler, president of the American Retail Federation, said his group was pleased with the Federal Reserve action because it "will show consumers the government's not trying to shut off credit."

Federal Reserve Chairman Paul Volcker has acknowledge complaints from retailers that the credit restraint program has had a psychological impact on credit users far greater than expected. Since March 14, retail sales have tumbled, with widespread reports that consumers thought the use of credit cards and charge accounts had become illegal or at least unpatriotic. oAdministration economists fear too large a reduction in consumer buying could guarantee a very severe economic slump.

Automobile loans, home mortgages and some other types of secured loans are not covered by the control. Revolving charge accounts, gasoline and travel credit cards and unsecured personal loans are covered.

In another step that had more significance for the nation's financial markets, the Fed also cut from 15 percent to 7 1/2 percent the special deposit requirement that applies to increases in the size of money market mutual funds and similar institutions.

For money market funds, the new requirement will be effective in the week beginning June 16.

The special deposit requirement caused many money market mutual funds to create "clone" funds into which new shareholders were channeled. The new funds, subject to the special deposit requirement, have been paying lower interest to shareholders than the older funds. Now that differential should be substantially reduced.

The Fed also:

Reduced the marginal reserve requirement on managed liabilities of large member banks and agencies and branches of foreign banks from 10 percent to 5 percent, and adjusted upward by 7 1/2 percent the base upon which the reserve requirement is calculated.

Reduced the special deposit requirement on managed liabilities of large non-member institutions from 10 percent by 5 percent, and raised the base by 7 1/2 percent.

Modified its voluntary "Special Credit Restraint Program" to ensure that more urgent credit needs are being met, such as those for small business, auto dealers and buyers, the housing market, agriculture and energy products and conservation.

Managed liabilities include funds obtained by banks from selling securities to a person with a firm agreement to buy them back at a later date.This had become a major source of lendable funds to banks prior to March 14. Money obtained this way was not considered a deposit and therefore not subject to normal reserve requirements.

In a letter sent to all banking institutions, the Fed said that increases in lending by small banks as a group have been well within its voluntary 6 percent to 9 percent guideline. "Those banks should not feel under any special restraint in meeting the needs of their regular local customers . . .," it said.

The Federal Reserve announcement stressed, "These actions do not represent any change in basic monetary policy as reflected in the targets for restrained growth in money and credit over 1980 that were developed early this year to help bring inflation under control."

In recent weeks, growth of the money supply has fallen well below Fed Targets because of the spreading recession. Analysts said yesterday's moves were undoubtedly prompted in part by a desire to get money growth back on track by giving a boost to some lending activities as well as to consumer buying.