A sharply divided D.C. Zoning Commission last night approved with minor revisions construction of the massive Techworld trade mart for high-tech products across the street from the city's Convention Center, including a controversial passageway bridge over Eighth Street NW.

The commission voted 3 to 2 to approve the $240-million complex, rejecting arguments by the Smithsonian Institution, preservationists and others that the project would spell the ruin of L'Enfant's 1793 plan for the development of the nation's capital by allowing construction of the five-story-high bridge starting 75 feet above Eighth Street.

The commission approved almost all of the plan submitted by developer Giuseppe Cecchi, who built the Watergate complex, but ordered him to make a series of design changes that would slightly scale back the bulk of the project.

The changes included cutting the proposed six-story bridge back to five floors in height and trimming some space from the top two floors of the 130-foot structure.

Cecchi, while voicing delight at winning approval of the project, said the "conditions in some respects are onerous and difficult. But they're workable, and I'm looking with optimism that we can work them out."

But he said that the required changes would force his firm, International Developers Inc., to postpone the start of construction from June or July until later in the year. Construction on the two-block complex, bounded by Seventh, Ninth, I and K streets NW, is expected to be completed in four years.

City officials, including Mayor Marion Barry, have staunchly supported Cecchi's complex, which includes a 910-room Ramada Inn, as another keystone in the redevelopment of the city's old and deteriorated downtown sector surrounding the two-year-old Convention Center.

Cecchi's goal is to turn more than 1 million square feet of permanent display and related office space into what he calls "a complete one-stop store for high-tech products," a relatively new concept that has been started with mixed success in Dallas. But, Cecchi said, "I feel better than ever about this project."

The zoning commission held more than 50 hours of hearings on the Techworld proposal since November and discussed it for another nine hours Monday and yesterday before voting to approve it.

In the end, the commission's chairwoman, Maybelle T. Bennett, said the panel was "either going to be part of the solution in revitalizing the old downtown sector or part of the problem in rejecting the proposal . It will provide personal opportunities for generations to come."

Cecchi has estimated that the construction work will create 300 full-time jobs until the project is completed, with another 1,300 permanent jobs available when the trade mart and hotel are opened. In addition, International Developers has projected that the city will reap $15 million in annual tax revenues from Techworld.

Commission members Lindsley Williams and Patricia N. Mathews joined Bennett in voting for the project, while two federal officials, Architect of the Capitol George M. White and National Park Service official John G. Parsons, vocally dissented.

By 3-to-2 margins, White and Parsons lost preliminary votes in their bids to scale back the height of the building's roof to 110 feet and to keep Eighth Street, which the City Council and Barry have agreed to close at Techworld's request, at its present 100-foot width. Under Cecchi's plan, the closed street will become the center walkway and courtyard for Techworld, with a width of between 60 and 85 feet.

But White and Parsons objected most strenuously to construction of the bridge that will connect the two wings of the complex. The Smithsonian, the D.C. Preservation League and other critics have argued during the hearings that the bridged structure will obstruct the Eighth Street vista that L'Enfant envisioned in his master plan. The vista now extends from the University of the District of Columbia's Carnegie Library on K Street south to the National Museum of American Art on G Street.

Parsons said the view from under the passageway would make the library look like "a billboard at the end of the tunnel. You might as well paint the library."

White told the commission's three-member majority they were "making a decision that will affect this city for the remainder of its life. I say it's too much to denigrate the urban design of the city. You put people on the moon, you ought to be able to sell high-tech products without putting a bridge over Eighth Street."

Mathews said she did not like the bridge either, but added, "I'm not persuaded it needs to come out."

With White and Parsons again dissenting, the commission voted 3 to 2 to slice off the bottom floor of Cecchi's planned six-story bridge so that the vista would be less curtailed.

The commission's staff now has to draft formal statements on the various requirements imposed on Cecchi before the commission can ratify them.