Pan American World Airways, the largest U.S. international carrier, opened its formal campaign yesterday to win government approval to initiate service in seven domestic markets.

In petitions filed with the Civil Aeronautics Board, Pan Am also proposed what were described as the lowest fares of any scheduled airline.

Dan A. Colussy, Executive Vice President of the airline, said regular coach fares would be slashed by 25 percent and that a standby fare would be offered at half the regular coach price.

Pan Am is seeking CAB authority to carry passengers between New York and Los Angeles, San Francisco and Houston; from Miami to Houston and Los Angeles; and between Houston and Los Angeles and San Francisco.

Reduced fares could be offered on these routes because such flights would be linked to existing international services now operated or palnned, Colussy told reporters yesterday.

"The new domestic authority we seek would allow Pan Am to expand its international services for the U.S. traveling public and at the same time provide needed low-fare services in domestic markets," he added.

West Coast to New York flights would continue on to London, Frankfurt, Amsterdam, Tehran, Bahrain, and Istanbul, for example.

Pan Am wants to call its reduced domestic rate a "fair fare," Colussy said. Between New York and Houston, the proposed Pan Am coach fare would be $103 compared with the existing $137 one-way charge on U.S. airlines serving that market. Pan Am's proposed standby charge for the same route would be $69.

Proposed fares between New York and San Francisco would be $166 (or $111 standby) compared with the regular one-way coach charge of $222. Higher first class fares also would be available in all markets. Overall. Pan Am forecast it would add $19 million in profits during the first year of domestic service, reducing dependence on international traffic.

Pan Am's proposals yesterday included material presented earlier to Secretary of Transportation Brock Adams, representatives of the White House staff and the Departments of State, Justice and Commerce.