American Telephone & Telegraph Co., which late Friday was ordered to pay $1.8 billion to a competitor in the largest antitrust judgment ever, today was the most active issue on the New York Stock Exchange.

But most analysts said they anticipate that AT&T eventually will pay much less than the $1.8 billion a federal jury in Chicago awarded MCI Communications Corp. of Washington, that it will be a year or more before AT&T exhausts its appeals and that there is a chance that the decision will be reversed.

Investors apparently agreed with the analysts. Although AT&T was the most active stock on the Big Board -- with 955,900 shares changing hands -- it registered only a minimal change in price, closing at $53.875, off $1.125 from its Friday close. MCI, on the other hand, which trades in the over-the-counter market, closed at about $9 a share, up from its $7.625 close Friday.

"It's only the first round. What's important is the final round," said Harry Edelson of Drexel Burnham Lambert, Inc. "The immediate impact on AT&T's profits is virtually nil," said Ernest Liu of Goldman Sachs. But Liu said that even if there is no immediate impact on AT&T earnings, the decision will encourage MCI to proceed with another, potentially larger, antitrust suit it has pending against the phone company and will encourage other AT&T competitors to go forward with their own suits.

Meanwhile, U.S. District Court Judge John Grady formally entered the judgment against AT&T today.

At the 9 percent yearly interest rate that currenly covers judgments of this kind, MCI is earning $3 million a week, or $162 million a year, in addition to the $1.8 billion. If AT&T wins its appeal, MCI would earn nothing.

Within 30 days, AT&T will appeal the decision to Grady, and if Grady denies the appeal, then within 30 days AT&T would go to the 7th Circuit U.S. Court of Appeals. The circuit court then would come up with a schedule for the appeal, before the case is briefed. If the court of appeals upholds the jury's decision, the case probably will be appealed.

The entire process could take as long as three years, by which time MCI would have earned close to $500 million on AT&T's money.

MCI had asked for $900 million in damages. Apparently the jury reasoned that because it found AT&T guilty of two-thirds of the questions set down by Grady, it should award MCI two-thirds of the damages it requested. The $600 million judgment was trebled to $1.8 billion under the antitrust law.

MCI had charged that AT&T tried to choke off its entry into the long-distance telephone business between 1971 and 1974.

Drexel Burnham's Edelson said the case was "almost too complex for a jury.

Cases like these are conducted with an eye to winning the appeal."

He noted that International Business Machines Corp., which has been the target of many antitrust suits by competitors, has lost a number of suits in the first round, "but they have never lost an appeal." IBM, a part owner of Washington-based Satellite Business Systems, which plans to compete with both AT&T and MCI in communications services, was among the active stock on the upside today, gaining 1 1/2 to 60 3/8. Another major telecommunications firm, General Telephone & Electronics, which owns Telenet of Washington, was up 1 1/8 at 29.