The Montgomery County Council yesterday substantially altered the boundaries of its five councilmanic districts, rejecting the arguments of a group of citizens who claimed the northern region of the county would be left with little representation.

Proponents of the plan drawn up by the bi-partisan Redistricting Commission say it accurately represents population trends over the last decade that showed increases of more than 100 percent in western Montgomery and declines of up to 20 percent in southern portions.

But opponents such as Robert Sher, president of the Potomac Association of Presidents, an umbrella organization of civic groups, criticized the new boundaries because the upcounty area, which did not gain as much population as the west, is no longer ensured of its own resident council member.

Upcounty groups had hoped District 2, which they have always been part of, would be extended as far south as Rockville.

Under the plan adopted yesterday, however, the district, which once covered all of the north and northwestern sections, will be cut in half and District 4 will double in size and extend north to the Frederick County border.

The new councilmanic framework shows new Districts 1 and 5 (Bethesda-Chevy Chase and Silver Spring-Kensington) cutting heavily into the old District 3 (Kensington-Wheaton) because of population decreases. The new District 3 has been shoved north and now encompasses Rockville.