Record high numbers of scaffolds across the city may be an eyesore for pedestrians and a headache for shops under their shadow, but the rise in sidewalk sheds underneath the scaffolds has also spurred a cottage industry in corporate advertising.

From a plug for iPods in Greenwich Village to a Helio display in SoHo to the Citibank sign snaking around the Flatiron building, billboard-style ads have cropped up increasingly on protective sheds in Manhattan and elsewhere, much to the dismay of civic groups and elected officials.

The ads are all apparently illegal under city rules, and the president of Manhattan, Scott Stringer, is launching a campaign to crack down on the practice.

“We’ve had enough of illegal black market advertising in our city,” Mr. Stringer said at a news conference outside the Flatiron building yesterday.

Businesses underneath a sidewalk shed may put up a sign for their store on the facing of the shed, but Section 27-03 of the rules governing city agencies appears to prohibit any other advertising on the sheds. Defining a sidewalk shed as a protective structure, the clause states that other than the business signs, “there shall be no information, pictorial representations, or any business or advertising messages posted on such protective structures at demolition or construction sites.” Almost all sheds are stenciled with the words “Post no bills” on the facing.

Citing data gathered by the Municipal Art Society in a survey of 42 illegal building and scaffold ads in the city, Mr. Stringer said 79% of the buildings had never been issued a violation for the signage.

“There is no oversight, there is no control, and no enforcement,” Mr. Stringer said, calling the ads a “blight” that the city should treat as a form of corporate graffiti.

The advertising ban has long been flouted, owing largely to a lack of enforcement and the fact that the revenue generated by the ads greatly exceeds the fines for violations. While Buildings Department fines for illegal advertising can range from $2,500 for an ignored first citation to $10,000 for ignored subsequent violations, one building industry executive said a billboard-style ad on a sidewalk shed can bring in up to $50,000 a month.

Since permits for sidewalk sheds typically run for a year, building owners often keep them up long after repairs are completed in order to collect revenue from the ads, officials said.

Mr. Stringer placed the blame squarely on the Department of Buildings, but to a certain extent, the agency’s hands are tied. City officials said that under current law, the department cannot unilaterally raise the fines for violations, and it cannot remove most signs outright without demonstrating that they pose a threat to public safety.

A spokeswoman for the Buildings Department, Jennifer Givner, said the agency was considering new enforcement strategies and was open to working with lawmakers and other officials to stiffen the penalties for illegal advertising and to give the department more power to remove the signs.

Ms. Givner also pointed to a new department initiative aimed at cracking down on illegal advertising near highways and parks. Under regulations announced last month and set to take effect this week, the city is creating a registry for outdoor advertising companies, which must provide a list of ads within 900 feet of a highway or 200 feet of a public park. Companies that violate the rules or that do not register their signs within 60 days face fines of up to $25,000 a day, which are designed to trump the likely revenue of the ads. The city will also have the power to remove those signs and can prohibit violators from bidding on city contracts for up to five years.

Still, the new program is unlikely to have an immediate impact on the proliferation of apparently illegal ads in dense neighborhoods of Manhattan and Brooklyn, where a construction boom and tighter building-code regulations have led to record levels of scaffolding and sidewalk sheds.

Robert Julavits, a spokesman for Citibank, which bought the ad wrapped around the Flatiron building, said yesterday the company would look into the matter and was committed to complying with any restrictions.