Trading was furious in American Telephone & Telegraph Co. and International Business Machine Corp. stocks today following the Justice Department's Friday decision to drop its antitrust suit against IBM and allow AT&T to split off its 22 local subsidiaries to settle a seven-year-old federal antitrust suit.

AT&T rose about $2 a share, while IBM, after an initial surge, rose only a quarter. But stocks of competitors of IBM and AT&T dropped sharply.

Analysts agreed that the split-up of the giant utility will be beneficial to the AT&T parent-- which retains its long-distance division, its Western Electric manufacturing subsidiary and its vaunted research laboratory but will be able to enter new fields free of federal regulation.

Trading in AT&T stock was halted Friday and could not resume until nearly 1 p.m. today because of a huge influx of buy orders. When trading resumed, 1.3 million shares of AT&T stock changed hands immediately at $60.625 a share, up $2 from its last trade Thursday. It was the first time AT&T stock had traded above $60 a share since last summer. By the end of the day, in which the Dow Jones industrial average slid more than 16 points, AT&T held firm at $60.625 on a composite volume of 2.56 million shares.

Although the federal government dropped its 13-year-old antitrust suit against IBM on Friday in probably the busiest antitrust day in U.S. history, the giant computer manufacturer closed $57, up 25 cents from its Friday close.

Trading in IBM also had been halted. When trading resumed on the New York Stock Exchange at about 1:45 p.m., IBM immediately jumped $1.75 on a trade of nearly 715,000 shares. But by the close of trading on the Pacific Exchange this afternoon, IBM settled back to $57. On all exchanges in the United States, trading in IBM totaled 1.8 million shares.

Competitors and potential competitors of IBM and AT&T suffered. Amdahl Corp., the biggest maker of so-called plug compatible computers (those whose programming can be used in conjunction with IBM computers) dropped nearly $2 a share. Washington-based MCI Communcications, which runs a long-distance service that competes with AT&T, was down $3 a share in the over-the-counter market. Western Union dropped $3.25 a share, and a company spokesman attribtued the decline to AT&T's settlement with the Justice Department.

"If you're in this industry and your name isn't AT&T, then the price of your stock is down," a Western Union spokesman said.

Analysts say that the new AT&T, which virtually will be free of regulation, will be able to enter new growth markets in telecommunications and data transmission that it was restrained from entering before.

Under the settlement with the Justice Department, the operating subsidiaries, such as the Chesapeake and Potomac Telephone Co., will not be able to engage in unregulated activities. They will become much like local electric utilities.

As a result, the outlook for the 22 operating subsidiaries is cloudier. Although their combined assets total $80 billion, about twice the assets that AT&T will retain, they will operate in a segment of the communications industry that will grow less quickly than other segments.

"Providing local phone service is a 'mature' industry," said one analyst for a major firm who asked not to be identified.

Since the financial health of the individual phone companies varies, their future is cloudier than the parent's in the eyes of most analysts.

"It depends on what form the spin-off takes," according to Mark Luftig of Salomon Brothers Inc., the big inevstment banking firm. With the exception of Pacific Telephone and Telegraph Co., the West Coast phone company, all the Bell operating subsidiaries have the top credit rating from investment services such as Moody's and Standard & Poor's. Although the companies sell bonds individually, the umbrella of the parent AT&T keeps their ratings high. Those credit ratings were not changed today.

But if AT&T decides to split off each company individually that would change. While Illinois Bell or Indiana Bell would keep their top rating, Michigan Bell might slip a notch and Pacific might descend several grades.

"My guess is that AT&T will do it on a regional basis," said Gerald Morgan, vice president and utility analyst at Bache Halsey Stuart Shields, a major brokerage firm. He noted that AT&T already had talked of grouping its operating companies on a regional basis.

For example, Morgan said, Michigan Bell by itself probably would not be a lucrative holding but if it was part of a Great Lakes company that also included Illinois Bell, Michigan, Indiana, Ohio, Wisconsin and Northwestern, investors would be more likely to keep the stock and bond ratings probably would not suffer.

But analysts cautioned that although it will be at least 18 months before the spinoffs occur, Congress, which has a communications bill before it, could change the scope of the antitrust settlement.

Many analysts suggested that with the "monkey" of a costly federal antitrust suit off its back, IBM would become more aggressive and hurt not only the new so-called plug compatible companies that depend on IBM for their existence, but the companies that make mainframe computers in direct competition with IBM--such as Sperry Univac and Honeywell. IBM has about 77 percent of the so-called mainframe computer (the unit that is central to a data processing system) market, with plug companies accounting for 12 percent and noncompatible companies 11 percent.

But George D. Elling of the brokerage firm Bear Stearns thinks that it is "nonsense" to expect IBM to become more aggressive. Since the giant began winning private antitrust suits in the late 1970s, most of which were based on allegations in the federal suit, IBM became "ultra-aggressive." He said the "tiger in the shadows" is AT&T not IBM.

If AT&T decides to venture into the computer field, presumably into niches that IBM is not in, a "lot of smaller computer companies" could be hurt, he added.