MCI Telecommunications Corp. asked a federal court here yesterday to order the dissolution of American Telephone & Telegraph Co. System for alleged discrimination and monopoly control of intercity telephone serice in the United States.

The sut, which also seeks roughly $3 billion in treble damages under federal antitrust laws, extends the time period of a separate suit MCI brought in 1974 against AT&T. That suit, which is pending before a U.S. District Court in Chicago, charges AT&T with monopoly control from 1971, when MCI began doing business, to 1974. The new suit brings the allegations up to the present.

MCI offers intercity microwave communications to customers between 50 American cities. The company does about $120 million in business a year and has been authorized by the Federal Communications Commision to expand operations into a total of 112 cities.

Although MCI provides its own means of transmitting signals from city to city, via a network of microwave towers, within each city it is basically dependent upon local telephone companies to provide the telephone hook-ups, usually through existing systems, to MCI customers.

MCI's suit alleged that local phone companies, at AT&T's direction, charge MCI higher rates and make service available on a more restricted basis to MCI than what is available to AT&T's long lines division.

The suit also charged that AT&T's pays unnecessarily high "settlements" to the seven leading independent telephone companies or groups that operate networks in this country as part of a conspiracy to keep the alleged AT&T monopoly secure. The seven, named as co-defendants in the suit, are General Telephone & Utilities, Corp., Continental Telephone Corp., the Lincoln Telephone and Telegraph Co., Mid-Continent Telephone Corp., Unitred Telephone Association.

In addition to asking the court to order AT&T kto divest itself of its 24 local telephone companies, MCI's suit also seeks the breakup of the longlines division of AT&T.

Depending upon the volume of calls a customer makes, MCI offers three different services, which permit a customer to make long-distance calls to cities where MCI has termi* nals. The lines can be, and often are, also used to allow computer-to-computer communications.

The Supreme Court left standing last November a ruling by the U.S. Circuit Court of Appeals for the District of Columbia ordering AT&T to make available local connections for MCI's Execunet, one of three services available to MCI customers.

A spokesman for AT&T said yesterday that MCI's suit "has absolutely no merit to it. We have not seen violating and are not violating the antitrust laws."