Mulitmillionaire Joe L. Allbritton filed a countersuit yesterday against efforts to thwart his takeover of Riggs National Bank, raising questions about loans to bank directors and hinting at management impropriety.

In the latest round of what is becoming an increasingly nasty fight over control of the city's largest financial institution, Allbritton charged in his counterclaim against the bank that management's resistance to his takeover efforts may be motivated by a fear of revelations that might bring scandal.

Allbritton, Riggs' largest shareholder with 15 percent of its stock, is opposed by the bank in his effort to buy enough stock to increase his holdings to approximately 35 percent. Last week Riggs won a temporary restraining order putting Allbritton's attempt to seize control of the 145-year-old bank on hold until a court hearing next week. Yesterday Allbritton countered by asking that the order be dismissed and that Riggs be forced to make certain disclosures.

Allbritton also asked for postponement of a company-set deadline today that may be critical to his long-run success. Riggs directors have scheduled a shareholders meetings for April 22 to vote on a proposal for a bank holding company that Allbritton has suggested could impair his ability to control the bank.

Only shareholders of record as of today are eligible to vote on the proposal. As a practical matter, shareholders who have agreed to sell stock to Allbritton probably would allow him to vote their shares. Nonetheless, the counterclaim asks that the record date be postponed.

Allbritton also reiterates claims that Riggs management, after initially welcoming his investment in the bank, inexplicably changed their minds and threw roadblocks in the way of his attempt to increase his holdings. On Feb. 9 Allbritton offered to buy at least 600,000 shares of Riggs stock for $67.50 a share, well above the $50 at which it had been trading the week before the offer.

"Threatened by Allbritton's challenge to their absolute power of control over Riggs and its affairs, the Riggs board and management commenced an unlawful program to thwart the offer," according to papers filed by Allbritton's attorneys yesterday in U.S. District Court.

The counterclaim charges that a statement issued by Riggs on Feb. 17 opposing the Allbritton offer failed to disclose sufficient information about loans made to directors.

"Riggs admits that it has made loans to at least seven of its directors or their associates -- loans totalling in excess of $5 million to each person, one as much as $23 million -- but fails to disclose sufficient information for Riggs shareholders to determine whether . . . these loans (comprising a peak total in 1980 of 47.3 percent of Riggs' total equity capital accounts) or officers of Riggs compromise the financial stability of the bank and reflect adversely upon the ethical standards of the Riggs board and management," according to the countersuit.

The counterclaim also charges that Riggs failed to disclose sufficient information about the terms of an office space leased by Riggs from a partnership in which Riggs director Oliver T. Carr Jr. has a major interest.

The information in the counterclaim is essentially a rewrite of information in the bank's "preliminary prospectus/proxy statement" filed with the Securities and Exchange Commission. The statement details loans to directors and notes in boilerplate language that they were made "on substantially the same terms, including interest rates, collateral and repayment terms on extensions of credit, as those prevailing at the same time for comparable transactions with others" and avers that the transactions represent no special risks. Although the counterclaim raises questions about the loans, it offered no evidence contradicting that claim.

The largest liability is that of Robert H. Smith, president of Charles E. Smith Building Corp., who had outstanding debt of $23 million at 13.3 percent interest. Richard E. Marriott, director and group vice president of Marriott Corp., owed $16.4 million at 10 1/2 percent, and Oliver T. Carr Jr., president of Oliver T. Carr Co., owed $15.9 million at 10.2 percent.

The proxy statement also says about the lease of office space from a partnership in which Carr is general partner that "in the opinion of management the leased contain the same terms and conditions as would have prevailed had they been negotiated with an unassociated person."