A group of 29 black truckers wheeled their rigs onto the construction site of the Washington Convention Center at 8 a.m. yesterday, and proceeded to block the haulting of dirt and rubble from the project to protest a Hispanic contractor's use of a white subcontractor on the job.

For nearly four hours, the protesters left their heavy-duty trucks parked on the now barren downtown land which the city hopes will be filled with big-spending conventioneers by mid-1982.

More than two dozen policemen led by Deputy Chief Robert W. Klotz -- an experienced negotiator -- warily eyed the protesters and eventually warned them that their trucks would be impounded if they didn't drive them away.

The protesters claimed that the Hispanic contractor, the Santa Fe Construction Co., was subverting the spirit of the District's minority contracting law by hiring a white-owned Fairfax County firm, A. G. Van Metre Jr. Inc., to do the hauling work.

As a result, the demonstrators said, small minority hauling firms, such as their own, were left without work that they could perform. The city law is designed to provide more jobs for minorities.

But half an hour after police threatened to start towing the protesters' trucks, the demonstrators finally relented and drove the vehicles off the dusty site near the intersection of 11th and H Streets NW.

Three gray Van Metre trucks that had been filled with dirt during the morning-long impasse then rolled out of the site toward a Virginia dump and bulldozers started loading other trucks.

"They're moving and I'm happy", said Klotz.

But the question raised by the truckers -- whether city-financed work specifically designated for a minority firm, and awarded to a legitimate minority firm, could then be subcontracted to a white firm -- kept several D.C. human rights, minority business and general services officials busy for several hours.

By the end of the day, Courtland V. Cox, director of the city's minority Business Opportunity commission, and Santa Fe President Jose Figueroa, reached an agreement: Santa Fe will attempt to hire more minority workers to augment those already being used by Van Metre.

Under city law, minority firms are entitled to get at least 25 percent of the money the city government spends on construction, service and purchase contracts.

Fred Bradley, the owner of a small trucking firm and a leader of the protesting members of the Metropolitan Truckers Association, said the group had no quarrel with the awarding of the $169,000 contract to the four-year old Santa Fe firm.

But he and Milton G. Carey, president of the Associated Minority Contractors, a trade group, both said that Santa Fe was undermining the spirit of the minority contracting law by winning the contract and then hiring a white firm to do the work.

"That clearly warps the hell out of the law," Carey said. "There is a sheltered market (requiring the hiring of minority firms) to deal with serious problem," the lack of sufficient minority business enterprise.

The practice of minority firms giving their work to white firms "simply has to come to a halt," Carey said.

Figueroa "represented that he was going to do all the work himself" when he got the contract, Carey said. Hiring a white-owned firm "places his affirmative action plan in jeopardy."

Figueroa, accompanied by his lawyer, Kenneth B. Weckstein, said that he had to subcontract the work because his company has only three trucks, and all three are in use on other construction sites. Figueroa said than Van Metre offered to do the work at the lowest cost.

He said that on a typical day there are seven blacks and three women among the 15 truck drivers hauling the debris away from the convention center site, although they all are hired by Van Metre.