Key executives from the financial services industry yesterday opposed any moratorium on the mergers of banks and nonbank companies.

"That horse is long since out of the barn," Walter Wriston, chairman of Citicorp, told the Senate Banking Committee. "To freeze this picture is, in effect, to allocate markets to those companies shrewd or lucky enough to be first off the mark."

Wriston and the chief executives of American Express, BankAmerica Corp., Beneficial Corp., Merrill Lynch and Sears Roebuck all argued that a six-month ban on the acquisiton of "nonbank banks" by securities and nonfinancial companies anxious to get into the banking business was essentially unnecessary, unworkable and unfair.

A nonbank bank gives up either making commercial loans or taking deposits to avoid regulation by the Federal Reserve Board.

Robert A. Beck, chairman of Prudential Insurance Co., was the only executive at yesterday's hearing to back the moratorium idea as a way of giving Congress time to do a comprehensive review of the banking implications of the mergers that are taking place.

The moratorium, originally proposed by the comptroller of the currency last April, is backed by the Federal Reserve system. At the request of Fed Chairman Paul A. Volcker, legislation was introduced in both the House and Senate last week to plug the nonbank bank loophole in the Bank Holding Company Act. The bill also would stop banks from getting into unauthorized activities such as selling insurance by buying a state-chartered bank in a state such as South Dakota, which allows banks to offer insurance. American Express Chairman James D. Robinson yesterday referred to this new regulatory game as "rent-a-state."

The legislation furthermore would halt an attempt by the Federal Deposit Insurance Corp. to allow state-chartered banks to underwrite securities through another loophole as well as restricing interstate banking. Forty percent of the states either have passed legislation or begun to consider legislation allowing interstate banking.

The moratorium appears to have little support in either the House or Senate banking committees as a substitute for a permanent solution to the situation.

Meanwhile, the Reagan administration plans to present its own bill for expanded powers through bank holding company subsidiaries to Congress before the Fourth of July recess. The purpose of the bill is to allow banks to undertake additional, riskier lines of business without endangering their soundness. Although academic witnesses testified last week before the committee that it is impossible to isolate a bank from its subsidiaries, the big bankers yesterday gave it their support. It is not clear whether the Treasury and Fed bills will be linked.

At yesterday's hearing, a majority of the industry chiefs backed interstate banking and risk-related federal deposit insurance. Wriston of Citicorp, Edward R. Telling of Sears and Samuel H. Armacost of Bank America Corp. also supported outright repeal of the separation of commercial and investment banking contained in the Glass-Steagall Act, even though it is still a political impossibility.