In a move that may directly challenge the success of its low-cost competitors in long-distance services, American Telephone and Telegraph Co. yesterday asked the Federal Communications Commission to let it experiment with new low-cost rates for off-peak hours and weekend long-distance calls.

"These requests are not being made in specific response to the competition," said Pic Wagner, an AT&T spokesman. "But there is competition in the marketplace. What we're saying to the commission is that deregulation is in order." That is, AT&T wants the same flexibility in pricing as its long-distance competitors such as MCI Communications Corp. and Western Union Co.

Essentially, AT&T wants to explore a variety of different rate plans that incorporate a basic flat monthly charge plus reduced charges for off-peak traffic. These experiments are expected to take place in parts of Iowa, California and New York, with the monthly charges for subscribers participating in the experiment ranging from $2 to $16. One plan would offer a further 20 percent to 50 percent discount on nightly and weekend rates; another would charge a flat hourly rate during those hours; and a third would provide for an allotment of calls with all calls after that allotment being billed at an hourly rate.

AT&T is asking the commission to waive requirements for providing detailed cost and market analysis. "By granting this request for waivers, the commission has the opportunity to speed the benefits of lower prices and wider choices," said Sam R. Willcoxon, designated executive vice president for marketing for the new AT&T organization that will provide long-distance services after the Bell System divestiture occurs January 1, 1984.

Spokesmen for the FCC and MCI Communications Corp. were not immediately available for comment.

AT&T also requested permission to expand the availability of its satellite digital circuits service and of its high-speed switched digital service. The former service allows users to create private line telecommunications networks for their own use. The latter service is principally for video teleconferencing applications.

By expanding both the range of the service and its potential customer base, AT&T would move into direct competition with such telecommunications network companies as Satellite Business Systems, the McLean-based venture between IBM, Aetna and Comsat. Western Union Co. also offers video conferencing services.