Treasury Secretary Donald T. Regan, testifying yesterday at the opening of congressional hearings on whether to overhaul the nation's banking laws, applauded a moratorium that has been imposed on the creation of "non-bank banks."

The moratorium, which will be in effect until Jan. 1, was imposed late Tuesday by Comptroller of the Currency C.T. Conover to give Congress time to consider the controversial issue of "non-bank banks." The term refers to banks that sidestep federal restrictions by declining to make commercial loans, one of the two activities that legally defines a bank.

The Federal Reserve Board, which has been at loggerheads with the comptroller's office over the issue, called the moratorium a "constructive move."

Regan, appearing before the Senate Banking Committee, said, "If the regulatory agencies were to supersede the job of Congress, I'd be aghast. I'd rather see Congress handle this than see it decided piecemeal" by regulators.

Regan, the administration's chief advocate of deregulation, was the lead witness at the first of a series of congressional hearings to determine whether or how to modify prohibitions against mixing commercial banking and the securities business as well as rules against nationwide banking. The process is expected to last at least two years, according to Senate Banking Committee Chairman Jake Garn (R-Utah).

In addition to the moratorium, Regan made the following points:

The Depository Institutions Deregulation Committee, which he chairs, is likely to lift all remaining interest rates ceilings on bank certificates next June.

Interstate banking should be permitted in all 50 states, not just in neighboring areas.

Allowing banks to enter fields like real estate and insurance through holding company subsidiaries will permit them to compete with other financial institutions while protecting the banking system.

The administration believes banks are "special" institutions because they use their funds for the benefit of the economy as a whole and should be treated differently from other commercial enterprises.

So long as banks are protected by federal insurance, the federal government should have the right to state what activities they may engage in regardless of state law.

The current legal definition of a bank is that it offers both checking accounts and makes commercial loans. By not making those loans, banks contend they can avoid the provisions of the 50-year-old Glass-Steagall Act, which separates commercial banking from underwriting securities. The Fed opposes this view, contending that it could undermine the safety of the banking system.

There have been at least 10 cases of "non-bank banks" chartered recently. Another four are pending, including one request to charter 31 national "non-bank banks" in 25 states. Though the pending applications will be processed, Regan quipped yesterday that they should be given "careful and lengthy hearing."

Without committing himself, Regan said that some members of the DIDC might support the removal of all remaining interest rate ceilings on time deposits. Federal Deposit Insurance Corp. Chairman William Isaac and Federal Home Loan Bank Board Chairman Richard Pratt have already proposed it, and Regan's vote would make it a majority of the panel.

The most popular instruments still regulated are the six-month, $10,000 minimum certificate of deposit pegged to Treasury bill rates and the small savers certificate, of 18 to 30 months maturity, with no statutory minimum.

In qualifying banks as "special" institutions, Regan said that just because they are "insulated" by insurance against the risks associated with other types of enterprise, they should not be "isolated" from affiliating with non-banking activities.

He said that the administration's policy is to allow many players into the game, but to require them all to play under the same rules. Bank holding company subsidiaries, whose activities would be watched by the same regulators who oversee similar activities by other companies, would bring about equality, Regan argued. As further protection, he would limit the percentage of bank holding company assets that could be invested in a given activity, such as real estate.

The only change in this year's version of the administration's bank holding company deregulation proposal is an exemption for banks with assets under $100 million. To save money, these small banks could act under their own subsidiaries without forming holding companies.

As for states permitting activities prohibited by federal law, Regan said, in effect, that there should be a single law for federally insured banks. He cited the case of South Dakota, which recently decided to permit state-chartered banks to write insurance. This move could prompt many states to change their laws to gain business.

Regan once favored easing into nationwide banking by allowing cross-state activities in natural markets such as the Washington area. He said yesterday he now advocates allowing branching in all 50 states.

"The plastic card," said Regan, "is going to outdo Congress."

Garn acknowledged that nationwide banking is "already here" and rued what he called a six-or seven-year congressional lag.