Former Washington Star publishers Joe L. Allbritton's plan to acquire the largest single block of shares in Riggs National Bank, the area's largest financial institution, apparently has hit a snag. It was not clear yesterday how serious the snag is, but a spokesman for Allbritton said the deal "has been postponed" during recent days.

Dynalectron Corp. Chairman Jorge Carnicero, who reached an agreement last month to sell 397,545 shares to Allbritton at a premium of $67.50 apiece -- about $30 million -- emphasized in a telephone interview from New York yesterday that the proposed sale has not been cancelled. He declined further comment.

Allbritton already own about 2 percent of Riggs common stock, and acquisition of the Carnicero-controlled shares would bring his share to 15.4 percent of the stock outstanding. Allbritton reportedly was expected to buy enough additional shares to bring his interest in the capital city's largest bank to about 25 percent.

Stock brokers and bankers interviewed yesterday speculated that Allbritton may be stymied in his effort to buy additional stock, that he may be having difficulty getting Federal Reserve Board approval of the transaction or that he was unable to reach agreement with Riggs management in discussions last week on directors he might nominate for the board of directors to represent his interests. Under banking laws, he could vote all of his shares for specific directors and elect three board members, as did Carnicero.

Riggs Chairman Vincent Burke Jr. said yesterday that any information on the Carnicero-Allbritton deal would have to come from those two men. He declined further comment, and Allbritton, a former Texas banker, did not return a telephone call. Other sources reported that some owners of smaller but significant blocks of Riggs stock have been buying additional shares in recent weeks. If the proposed sale to Albritton falls through, Carnicero is expected to sell his interest to another investor; he has been approached by such potential buyers in the last year.

Stocks of major regional banks, including Riggs and arch-rival American Security Corp., are being sought after by investors because of expected moves by government to relax some interstate branching laws. The Washington area has been pinpointed as a possible test mardet for allowing D.C., Maryland andVirginia banks to open branches across state lines.

The stock of American Security, which owns American Security Bank, has jumped 1o percent in the last month and 7 percent in the past week to a new 52-week high of $38.75 a share. Volume last week was an unusually high 136,000 shares, and at least one broker said Allbritton may have switched his interest to American Security, where he served as a director during the period when he owned The Star. Others attributed the ASB pruchases to large institutions. Riggs stock is up 5 percent in the past month to $51, off its recent high of $53 that followed the Allbritton-Carnicero agreement.