If you're strolling through the Old Port today, you might notice a message scrawled on a dirty car window, painted on a rock plunked on your lunch table or stamped on the sleeve of a passerby's coffee cup. "This ad cost nothing" it might read, or "The less we spend on advertising, the more people we can reach." If you wind up donating to The Salvation Army after seeing it, consider yourself a successful target of "guerrilla marketing."

Local ad agency The VIA Group began blanketing Portland, South Portland and Westbrook on Monday with a pro bono grassroots ad campaign on behalf of The Salvation Army, designed to boost awareness and donations during a typically slow time of year for the charitable organization. Roughly 50 area businesses are donating space for the effort, including the Portland Pie Co., which will have ads stamped on the underside of its pizza boxes, and Novare Res, where messages will be painted on the bar's mirrors.

Volunteers from VIA and The Salvation Army took to the streets to disseminate the ads as part of an undertaking valued at about $250,000, VIA CEO John Coleman told Mainebiz. All told, the team executed 5,000 individual ads in various ways, says Coleman, who credits the agency's younger associates for taking the lead on the effort. "This campaign was not just like making a print ad and calling the paper and sending it over," he says.

The campaign also includes traditional ad space donated for print, online and television ads. The goal was to keep costs down while underscoring the charity's message that 83 cents of every donated dollar benefits programs and services, Coleman says.

Increasing visibility and donations without creating a perception that donor money was funding the campaign was a challenge, according to Craig Evans, director of development for The Salvation Army's northern New England division. "The Salvation Army has never invested heavily in any kind of marketing or advertising," he says. People are more aware of the organization during the holiday season thanks to its ubiquitous red kettles. But, like many other charities, it's struggling amid the tough economy, and donations through its direct mail appeals are down 20% to 30%, he says.

Founded by William and Catherine Booth in mid-19th century London with a mission to serve the destitute, The Salvation Army has its roots as a grassroots group. "For an organization that rose out of the streets, a guerilla campaign is really fitting," Evans says.