The recently proposed MBTA plan to extend subway service up Mass. Ave. was rejected by the Cambridge Advisory Committee Tuesday.

But the rejection may be temporary if the MBTA will agree to a CAC proposal to provide access to the Harvard Square station from the Brattle-Mt. Auburn St. area and the future Kennedy Library.

Frank J. Park, execuive director of the CAC contends that in the next five years thousands of people will be visiting the library and the Brattle-Mt. Auburn St. commercial district will be greatly expanded.

Access Tunnels

To ease this new traffic problem the CAC wants the MBTA to transform access tunnels to the Bennett St. MBTA yards (the JFK Library sight) into walk-ways leading to the main station under the Square. These tunnels pass beneath Brattle St. and could provide extra entrances to the subway for both shoppers and library visitors.

But the MBTA has so far refused to go along with this proposed change.

Under the present MBTA plan, Park noted, entrances to the subway would be along Mass. Ave. thus cutting off Brattle and Mt. Auburn Streets and the Kennedy Library from easy access to commuters. "We believe a much improved design can be made available which will serve the public better," he added.

James F. Farr, chairman of the Harvard Square subcommittee which studied the MBTA proposal said yesterday he thought the plan could have shown more imagination with respect of pedestrians."

For some time the University has worried about chaos Kennedy Library visitors might create if dumped directly into the Square. "We don't want 2.5 million people getting out in front of Holyoke Center." Charles P. Whitlock, Assistant to the President on Civic and Government Relations said yesterday.

During the next few weeks the CAC will be consulting with the MBTA and has offered to provide technical assistance to the MBTA to work out details involved in use of Bennett St. tunnels.

The subway plan would also continue the line up Mass. Ave. to Cambridge Common.

The Mass. Ave. plan was one of four plans submitted to the City by the MBTA. Two of the other plans would have run the subway under Harvard property -- one of them was to have gone under the Radcliffe Yard. Both were not considered acceptable by Harvard.