At 6:30 p.m. last Tuesday, Michelle Murrain showed up at a downtown Oakland, Calif., street corner to meet with 15 strangers who had organized themselves over Facebook. Many showed up with $20 bills.

Their mission was to descend on Marion & Rose’s Workshop, a gifts boutique, to spend money.

Ms. Murrain and her compatriots are among hundreds of devotees of the “cash mob,” a new social-networking-and-shopping movement aimed at increasing sales at selected small businesses.

Similar cash mobs have materialized in more than 20 cities from Norman, Okla., to Muskegon, Mich., most arranged by individuals who establish followings on Facebook and Twitter. The cash-mob organizers don’t get any benefit in return.

Lisa Gilmore: A ‘cash mob’ patronized Kellygreen Home in Santa Monica this month.

The Oakland event, for instance, was organized by Alex Haider-Winnett, a paralegal and participant in the Occupy Oakland movement.

Cash mobs are one of many buy-local campaigns that recently have spread to communities across the country. One in four business owners say poor sales is their top business problem, ahead of any other issue, according to a November survey from the National Federation of Independent Business, a small-business lobby group.

Last month, in an event called Small Business Saturday, American Express Co. provided its cardholders with a $25 credit if they used the card at small retailers the day after Black Friday. Hundreds of thousands of consumers registered their cards to participate in that promotion, according to American Express, the event’s sponsor.

In contrast, cash mobs spring up organically through social media outlets and have no corporate sponsor or formal advertisements. (At least one cash mob, in Grand Rapids, Mich., was organized by the local Chamber of Commerce, however.)

The first known cash mob was the brainchild of Chris Smith, an engineer for Oracle Corp. The 37-year-old from Buffalo, N.Y., says the idea stemmed from his realization that consumers, including his wife, tend to flock to smaller establishments when a bargain is available through the daily-deal social-networking sites including Groupon Inc. and LivingSocial Inc.

“Why do we need a discount to support good, solid, local businesses?” he asks.

He used Twitter and Facebook to rally more than 100 people to purchase wine at City Wine Merchant on Aug. 5. Business that day tripled, according the store’s president, Eric Genau. “We have clients that would have taken a lot longer to get here or never would have gotten here at all if not for that,” Mr. Genau says.

Several weeks later, a group of young professionals in Cleveland say they independently came up with the same concept and the same name. Their first event, on Nov. 16, drew about 40 people to a bookstore and, afterward, to a bar for drinks. They launched a blog, and the idea began to catch on in Albuquerque, San Diego and elsewhere.

In some areas, Occupy Wall Street affiliates have embraced the idea. The Cleveland group has distanced itself from that cause, noting on its blog that cash mob “isn’t a political or social organization … or meant to be an answer to economic crisis.”

Sarah Ditzenberger, owner of Fischberger’s Variety, a gifts store in Milwaukee, says one cash mob that showed up to spend money at her store earlier this month boosted sales by $1,200, roughly doubling an average day. The extra cash will help to pay off inventory and other business debts, she says.

Sales at the Oakland gift store last Tuesday were around $450, or about double the store’s typical sales. Ms. Murrain, a 52-year-old Web developer, said she purchased herbal tea and greeting cards during the Oakland cash mob event, spending $30.

Some cash mob groups are planning to continue their events in January and February.

Mr. Smith of Buffalo, who has organized mobs at a beer store, a restaurant, a bookstore and a coffee shop, plans to ask his Twitter followers to nominate a business for another cash mob to be held Friday, Jan. 6.