Yesterday began as just another day for silver-haired Clara B. Wallace, 81, until Mayor Marion Barry's office invited her to listen to a band and watch Barry sign a bill providing stricter penalties for certain crimes committed against the elderly.

Wallace, a widow who lives at the Capitol View Plaza apartments, 5901 East Capitol St., joined about 40 other elderly residents of public housing projects who were transported in D.C. Department of Recreation vans to the signing ceremony in Western Plaza across from the District Building.

It was the latest in a series of carefully orchestrated appearances by the mayor designed to solidify support among the city's 103,000 elderly residents before the Sept. 14 Democratic mayoral election.

While there are no official figures on the number of elderly voters in Washington, campaign aides to Barry estimate that 30,000 of the city's 280,000 registered voters, or 10 percent, are age 60 and over. Elderly voters are widely regarded as an important political force because they tend to faithfully turn out to vote.

Barry has courted this constituency with millions of dollars' worth of new city programs and subsidized housing units since taking office, and he now hopes to reap the political benefits.

According to a Washington Post poll published July 1, about 48 percent of registered Democrats age 60 and over said they supported Barry, compared to 21 percent for Patricia Roberts Harris, whom the polls showed as Barry's chief rival.

Sharon Pratt Dixon, Harris' campaign director, acknowledged yesterday that Barry may have a lock on the elderly vote because of his use of the incumbency. She said Harris will attempt to woo elderly voters with public appearances at housing projects and through a media campaign. But Harris has no plans to designate a campaign aide to work full time trying to organize senior citizens, as Barry has done.

"We can't engage Barry in every battle and try to best him on every front," Dixon said. "We will reserve our strength for when it matters."

Barry, who mingled easily with the senior citizens and businessmen who assembled under a green and white tent for yesterday's ceremony, said his support among the elderly is part of a broad-based constituency he claims to have assembled for the September primary election. "I'm going to do very well among all voters, but the seniors are very appreciative of my strong support," Barry said.

Barry increased the city's annual budget for programs and social services for the elderly from $652,000 when he took office in 1979 to $4.9 million in the fiscal year that begins Oct. 1. He mailed "Gold Cards" to thousands of senior citizens providing them with discounts at many stores. Also, more than 1,600 units of rent-assisted housing for the elderly were completed during his term.

All this was aimed at people like Clara Wallace. She said she appreciates the special attention that she and other elderly residents get from the mayor, who has visited more than 50 housing complexes for senior citizens and she intends to vote for him on Sept. 14.

"He comes to our building to talk to us and he gave us a telephone number to call if we need help," Wallace said. "As far as I know, no one has ever called and didn't get help."

Wallace said she also appreciates Barry's support of the crime bill, which was sponsored by City Council member--and candidate for council chairman--David A. Clarke (D-Ward 1). The measure revamps laws governing white-collar crime and consumer fraud. It also provides penalties of up to one-and-a-half times the maximum for robbing, extorting or defrauding persons aged 60 and older.

"There's such a fear . . . in the streets and in your home," Wallace said. "I'm so happy something will be done."

James H. Wilkerson, a Barry supporter who lives at Claridge Towers near Thomas Circle and attended the ceremony, said he regularly receives city services such as the use of recreation buses to go shopping. "He Barry is practically the head of the household," Wilkerson said.

Genevieve N. Johnson, president of the Greater Washington Area Council of Senior Citizens, with more than 200 clubs, said Barry took office with little understanding of the problems of the elderly, but has done much for them since then.