Transportation Secretary Brock Adams today announced almost $280 million in grants for mass transit in New York City.

The grants will help renovate and expand the most heavily used mass transit system in the nation. About 31 per cent of all mass transit riders in the United States are in the metropolitan New York area, according to a spokesman for the city's transit authority.

Adams said the grants "and others we will announce in the near future will serve to dispel the mistaken notion that the Carter administration is biased agains mass rail transit."

They were announced at a time when New York officials have been complaining about lack of response to the city's many problems from the Carter White House and they may also serve to ease those strains.

New York will use $102.3 million of the federal grant funds to continue its $500 million subway rehabilitation program and to purchase 325 air-conditioned, 53-passenger buses.

More than half the total will go for replacement and repair of old power, track, signal and communications facilities.

Other funds will be spent to extend the long Island Rail Road service into Grand Central Station, to complete further work on a new subway linking Manhattan and Queens under the East River and for a new station at Sutphin Boulevard in Queens.

A spokeswoman for the Transportation Department said the grants to New York were among the largest made under the Urban Mass Transportation Act.