Preparing for organized labor's Solidarity Day march in Washington Saturday, Robert E. Petersen described it as "a celebration" of the 100th anniversary of the organized labor movement in the United States, but admits that in his business, these are hardly festive times.

Petersen is president of the Greater Washington Central Labor Council, which comprises 150 unions in the Washington area and claims 250,000 members. He said that for all of them, "things are terrible."

Organized labor for decades has had difficulty making itself felt in the Washington area. Now economic conditions, legal trends and the policies of the Reagan administration have sapped the strength and morale of the unions, Petersen said.

Petersen, interviewed in the Labor Council's downtown office, disputed the traditional view that Washington is not a union town. Unions have existed here for more than a century, and with a quarter-million members, they represent a substantial portion of the work force, he said. But Petersen acknowledged that because members are scattered through three political jurisdictions and are involved in diverse occupations, unions here cannot hope to exert influence comparable to that of the auto workers in Michigan or the rubber workers in Ohio.

In addition, the labor movement here lacks influence proportional to its numbers because many union members are public employes, prohibited by law from striking, or federal workers, barred by law from partisan political activity. But the unions' current difficulties, Petersen said, are traceable not to their inherent weaknesses, but to the policies of the Reagan administration -- policies that union members from across the country will protest here this weekend.

"Things are going as terrible as they are now because of the Reagan policies, which to us, at least, seem to be extremely antiunion. We are on the defensive, trying to defend the things we fought to get for years," he said. State and local governments and businesses "take their lead from Reagan and say, 'The president is antiunion in his actions, let's get tough with the unions.' "

He said the administration's dismissal of striking air traffic controllers and refusal to negotiate with them intimidated other government workers.

"It's terrible that the president wants to do this in a dictatorial way. To them, collective bargaining means, 'We give a decision and you accept it.' That's not collective bargaining." But the policy has a ripple effect on state and county officials, he said. They say, " 'If Reagan does it, we can do it, too.' It will have a hell of an effect throughout the nation," Petersen said.

Petersen gave these reasons for labor's admitted failure to make more of an impact in the Washington area:

* Efforts to organize nonunion businesses are undercut by "a large number of antiunion consultants in this area, employed by many of the businesses to come and have the union decertified, or if it's a nonunion company, to educate the supervisory staff on how to eliminate any possibility that it would go union. You have notorious national people in this town, who come in and give lectures and seminars on how to keep unions out of the work place."

* The District of Columbia government, he said, has consistently given construction contracts to nonunion contractors. Even the convention center, the biggest current project, is being built mostly by nonunion labor, Petersen said. In addition, the city government, while engaged in collective bargaining with the unions over next year's salaries, is also handing over some of its functions, such as maintenance work at the University of the District of Columbia, to contractors who use nonunion workers.

* High rates of unemployment have so increased the labor pool that nonunion employers have no trouble hiring workers for jobs that might otherwise be union-controlled.

* Legal restrictions have undercut union solidarity. A union that pulls its members off a job to honor another union's picket line faces court action and heavy damage claims for violating its own contract.

Petersen said that was why the unions were unable to prevent New York Air from operating with nonunion help. The leaders of the machinists, electricians, baggage handlers and food workers unions objected to New York Air's policies, he said, but they did not walk off the job to honor the picket lines of the Air Line Pilots Association.

As a result, he said, unions here are in "a sad situation" in which "we don't have near the impact that we should have. I wish we did. All we're trying to do is raise the standard of living for the workers."

The unions say they expect as many as 100,000 participants in the Solidarity Day events. It will be an "economic bonanza" for local businesses, Petersen said, but it has a more long-range purpose: "To challenge the Reagan administration's so-called mandate to dismantle 50 years of social and economic progress."