The Federal Communications Commission is flexing its muscles for the first time in the court-ordered breakup of American Telephone & Telegraph Co., an exercise that could influence significantly the outcome of the divestiture process that began in January 1981.

The commission is asking AT&T to provide details on its plans to transfer nearly 2,000 radio licenses, which are affected by the company's proposed spinoff of its 22 operating companies into seven regional companies. The breakup is scheduled to take place by Jan. 1, 1984.

FCC action on those license-transfer plans could delay seriously or could change substantially the divestiture program worked out among the company, government lawyers and U.S. District Court Judge Harold H. Greene.

The agency has set a March 1 deadline for an initial AT&T response.

Until yesterday, when the agency's commissioners approved the letter that is expected to be delivered to AT&T today, the FCC had been relegated to a secondary role in the divestiture proceedings. The major action had been in the courts.

But the 1934 law that created the FCC gives that agency review and veto power in matters affecting the establishment, transfer or termination of transmission facilities and licenses.

"We've been on the sidelines for a long time in this thing," said FCC Commissioner Henry M. Rivera. "The letter represents the first time that the commission, itself, is exercising authority" in determining outcome of the divestiture, he added.

The FCC's primary goal in reviewing the license transfer plans is to determine whether any of the transactions will undermine emergency and defense communications systems. The agency also wants to know what effect, if any, the proposed transfers would have on ownership patterns and pricing in matters such as microwave communications.