A plan to convert the massive and underutilized torpedo factory on the Alexandria waterfront into a residential and commercial complex has been proposed by a Falls Church developer, but the idea already has been attacked by some conservationists and the director of the art center that now occupies part of the site.

The $6 million proposal by the Redstone Development Corp. envisions a partially redesigned torpedo factory that would include 236 one- and two-bedroom apartment units, parking space for 423 cars, as well as commercial development. The art gallery, a waterfront park and plazas with waterfront views also would be included.

In a letter to Alexandria officials, Samuel J. Rosentein, president of Restone, said the proposal will "contribute to the overall development of downtown Alexandria" and would generate more than $100,000 in new real estate taxes for the city. Rosenstein also said that the proposed 215-by-175 foot waterfront park would be donated to the city.

Alexandria City Manager Douglas Harman unsuccessfully has tried to convince the city council to formally ask developers for ideas about what to do with the torpedo factory buildings at the foot of King Street on the Potomac River shoreline. He said yesterday that proposals such as the one by Redstone should be considered seriously before a deciison is made to demolish the buildings, as has been proposed by some city council members.

In January the city council voted to demolish one of the four main buildings that make up the complex, a project that would have cost an estimated $800,000. The council reversed itself later and decided to hold a public hearing on th question next Saturday. Demolition is costly because the building, constructed during Work War I and expanded at the time of World War II, is strong enough to withstand small torpedo explosions.

Alexandria bought the torpedo factory from the federal government for $1.5 million in 1970. The 4 3/4 acre site is not part of the land title dispute between the city, private landowners and the federal government over ownership of other portions of the Alexandria waterfront.

City council member Ellen Pickering, a long-time proponent of a national historic park along the waterfront labeled the Redstone proposal "just one more example of the vultures preying upon the crown jewels of the city, the crown jewel being the waterfront."

Under the plan, a total of 12,000 square feet in one of th buildings would be allocated for use by artists for their exhibitions and studious, compared with the 48,000 square feet now being occupied by the Torpedo Factory Art Center.

"Although reduced in size and scope from the present day artistic activities this space would retain the essence and flavor of the artist workshop areas," Rosenstein said in his letter.

Marian Van Landingham, the center's director, said that to place all of the present activieis into such a small space "would in a sense be killing" the center and she predicted "a large public outcry" against that proposal.

Van Landingham said that even now she receives two to four requests a day from artists who want space in the complex, which now houses four nonprofit galleries and working space for 205 artists. In addition, an art school with 250 students meets at the center.

Besides the art center, the torpedo factory currently houses a variety of city offices, including a printing plant. One of the buildings is being used as a parking lot for 150 vehicles. In his letter, Rosenstein said that "more than adequate parking facilities would be contained within the (refurbished) buildings of the complex itself," with 63 of the 423 parking spaces made available to the general public. Parking has become a problem in Old Town Alexandria in recent months, a point that will likely be raised by those opposing the project.

The developers said they plan "selective demolition" of the buildings to reduce their scale. Walls of one building now being partly used as a parking lot would be removed, which would create an open-spaced plaza with a view of Carlyle House, the restored mansion where in 1755 five British governors met with General Edward Braddock and proposed the Stamp Act, which set off the cry of "taxation without representation" and the American Revolution.