Not an acre has been purchased and not a spadeful of dirt turned, but almost everyone who testified at yesterday's hearing on the proposed Washington Civic Center seemed to have a clear vision of the project's success or failure.

Gathering at the Martin Luther King Library just a block south of the proposed center site, a procession of witnesses argued with equal fervor that the complex would be a boon to the city's economy or, on the other hand, a gamble likely to backfire on D.C. taxpayers.

"The taxpayers are the risk-takers, and the business community is the only winner," declared John Kelly, staff director of the Upper Northeast Coordinating Council.

But R. Robery Linowes, president of the Metroplitan Board of Trade, said the center would bring an "infusion of new life into a secton of downtown that sadly lacks it."

Linowes stressed that the center - to be built on a three-block site next to Mount Vernon Square and bound to New York Avenue and H, 9th and 11th Streets NW - would create 4,000 sorely needed retail and service jobs associated with the convention and trade show business it is designed to attract. Kelly and other opponents of the project questioned whether such jobs would be forthcoming.

Yesterday's hearing came just two days after the city had disclosed scaled-down plans for a somewhat cheaper convention facility than the one previously rejected by a Senate subcommittee.

In order to win approval for the project, the city had been asked to reduce its costs, enact supplemental taxing measures to help pay for it and secure commitments from the business sector to build related "spin-off" development projects such as hotels, stores and restaurants.

City officials have so far failed to provide specific documentation of such projects, but they have promised a detailed accounting by the time the new center proposal is sent back to Congress in mid-September.

"I want to emphasize just how difficult it is to obtain such committments," said Linowes, an attorney who specializes in real estate law. "Individual investors are understandably reluctant to disclose, prematurely, their development intentions."

Many potential investors "are gunshy" because they lost money in 1973 when Congress failed to authorize construction of the then proposed Eisenhower Center, Linowes added.

The most visible opponents of the center project yesterday were members of the Convention Center Referendum Committee, a group of 80 community associations and 10 Advisory Nieghborhood Commissions who want the whole convention center issue place on the ballot.

Jack Phelan, a government economist and member of the group, challenged most of the convention attendance and revenue-producing estimates provided by the city in support of the project.

He said the city-hired consultant firm for the center, Gladstone Associates, has projected that the new complex would eventually attract between 31 to 38 large conventions.

This Phelan complained, contrasted sharply with another Gladstone study done for the proposed New York City convention center. That study showed that the most successful convention facility in the country, Chicago's McCormick Place, booked only six conventions all of last year while the Las Vegas center booked only 10.

Phelan also accused Gladstoe Associates of having a possible conflict of interest in its convention center estimates since it is a subsidiary of a firm that is helping to develop an 800-room hotel five blocks from the center site. The Gladstone firm earlier had denied any conflict, saying its operations are completely independent of those of its parent company, Quadrangle Development Corp.

The city is now "losing millions of tax dollars to other cities," which have facilities large enough to accommodate major conventions, said Leonard Hickman, executive vice president of the Hotel Association of Washington. Hickman urged that the project move ahead as swiftly as possible.

City Administrator Julian Dugas, in opening testimony favoring the center, said the city feels "we have met the conditions" for approval sought by the Senate subcommittee and hopes to get a go-ahead before Congress adjourns this fall.