With D.C. Mayor Marion Barry donning multiple hats as tour guide, cheerleader and politician yesterday, the office construction market and the economic development of downtown Washington were highlighted in a showcase event designed to promote the city.

Approximately 50 million square feet of privately owned office space is in downtown D.C., putting Washington third nationally in that category.

What's more, the office building boom shows no sign of abating as the supply of new space over the past three years has averaged just over 2 million square feet. Some local observers fear a glut will jolt that pace to a sudden stop in the next couple of years, especially now that the nation is beginning to feel the effects of a recession.

In the meantime, soaring rental rates--projected to reach $40 a square foot by 1983--haven't affected growing demand for the more than 3 million square feet of office space scheduled to come on the District market next year.

Only New York and Chicago surpass D.C. in downtown office space.

But the District's attraction as a major office market is one of its least publicized strengths, city officials have concluded. And admittedly smarting over attention being given Baltimore in the wake of that city's renaissance, Mayor Barry has decided to do a little drum-beating himself.

With the unlyrical theme of "D.C. on the Grow," Barry yesterday launched a campaign to emphasize the economic impact of various construction projects downtown.

With the mayor as host, about 60 developers, heads of construction firms, business leaders and city council members boarded a double-decker bus for a three-hour tour past 15 major building sites.

At each building site, posters identified the project, the economic impact and a reminder in bold print that the mayor strongly supports development and creation of jobs in the city.

With motorcycle police escort, the mayor's VIP tour bus weaved through downtown streets as an aide reviewed a set of impressive figures on economic development.

D.C.'s Office of Business and Economic Development estimates that the 15 projects, including the convention center, will cost about $566 million to develop but will generate more than 6,300 construction jobs, 11,800 permanent jobs and about $26 million a year in revenues for the District.

Although most of those on the bus seemed impressed by the mayor's promotional tour, there were those who wondered privately what undisclosed motives he might have. Could it be part of his overall campaign for reelection?

At one point during the tour, Lawrence Schumake, director of the Office of Business and Economic Development, felt it necessary to announce that the bus was passing the Metropolitan Police Department's District 2 headquarters in Southwest. The mayor, Schumake reminded those aboard the bus, has made crime prevention a major issue in his administration.

"Well, I guess that's touching all the bases," one passenger quipped amid a small chorus of laughter.

Asked about the rationale for the tour, Barry said it's only part of his overall plan to promote the District, but he acknowledged it could have some political implication.

"Most politicians, if they are honest, will tell you a lot of what they do has some political implication," said the mayor. "Sure this will benefit me, but the important thing is jobs. The bottom line is that people will benefit."

Barry's "D.C. on the Grow" campaign--an idea he conceived and promoted--will continue with a "similar thing on housing" and a visit to Anacostia for business and community leaders, he said. The entire campaign, explained Schumake, "is a step toward creating pride" among District residents.

"From the mayor's perspective," Schumake continued, "it's important that people see the same type of impact on the city as we see in Baltimore."

Schumake acknowledged under questioning that most of the projects included in yesterday's tour are being developed by the private sector. However, some projects have been "leveraged" by city resources, he pointed out.

As an example, he noted, a warehouse formerly owned by the city at 4th and D Streets SW soon will be developed by owners of Chicago's Merchandise Mart.

During remarks at an open-air luncheon amid concrete and steel girders at the convention center, Barry promised that he will take steps to ensure that commercial development east of 15th Street NW be carried out differently so that a more livable area would be the result, in contrast to downtown west of 15th.

In a windswept corner of the mammoth convention center project, Barry told his luncheon guests, clad in hard hats, that cities should be "dynamic, not static."

Stressing the importance of a strong marketing campaign as part of the city's economic development program, Barry promised "we're going to sell ourselves" and not be outdone by other cities.

Then, borrowing from the Bible, the mayor declared: "Let your light shine before men so that they may know your good works. We're going to let our light shine and continue to grow."

The luncheon was the first official event in the center, where the first convention will be held in January 1983.