engine "to highlight Bing's commitment to honest search results."
The campaign also seeks "to help explain to consumers the risks of Google Shopping's newly announced 'pay-to-rank' practice," a Microsoft statement said.
Microsoft created a Web page called "Scroogled," which points out that its rival has reversed course on its pledge at the time of the Google stock offering to avoid paid ad inclusion for search results.
"Google Shopping is nothing more than a list of targeted ads that unsuspecting customers assume are search results," Microsoft claims.
Google announced earlier this year it would revamp its product search to become a shopping service with paid listings. This eliminated merchants which opted not to pay, including some notable ones like Amazon.
Google said it completed the transition October 17 in the US, and will be rolling out the same model in Britain, Germany, France, Japan, Italy, Spain, Netherlands, Brazil, Australia and Switzerland.
"We think this will bring the same high-quality shopping experience to people -- and positive results to merchants -- around the world," a Google blog posting said.
Microsoft is promoting its campaign online and offline with ads "demonstrating why consumers should be concerned and helping them take action" on the Google shift, a Microsoft statement said.
"We're also calling on Google to stop this 'pay-to-rank' system for their shopping results and give shoppers what they expect -- an honest search."
Google maintains that merchants cannot improve their rank simply by paying more, and that sellers who have a financial stake in the results will keep their information up to date.
"Google Shopping makes it easier for shoppers to quickly find what they're looking for, compare different products and connect with merchants to make a purchase," said an email from a Google spokeswoman.
But some analysts say both companies are less than transparent about how their shopping engines work, and that Microsoft is not without blame.
Danny Sullivan, analyst with the website Search Engine Land, said of the Microsoft effort: "Great campaign, if it were true. It's not. Bing itself does the same things it accuses Google of."
Sullivan told AFP that "at least Google has the fine print that you can read; Microsoft doesn't have it at all."
Microsoft, according to Sullivan, excludes new merchants from Bing search results if they don't pay for inclusion with its partner, Shopping.com, even though this is not fully transparent to consumers.
"Payment is a factor for ranking," in Bing, said Sullivan, who maintains that Microsoft's campaign is misleading.
Microsoft said its own shopping results through Bing are not influenced by payment.
"Bing includes millions of free listings from merchants and rankings are determined entirely by which products are most relevant to your query," said Stefan Weitz, senior director at Bing, in an emailed statement.
"While merchants can pay fees for inclusion on our third party shopping sites and subsequently may appear in Bing Shopping through partnerships we have, we do not rank merchants higher based on who pays us, nor do we let merchants pay to have their product offers placed higher in Bing Shopping's search results."
Sullivan argues that Google, ironically, may have moved to paid listings to deflect attention from regulators and others who complain it had been skewing its search results.
"If you have people complaining you search results are unfair, you can turn them into ads," he said.
But Sullivan noted that Google merely adopted the same policies of most shopping sites, which use paid listings even if they appear to be an impartial search.
The overall message from the latest row, according to Sullivan: "You need to shop around. Use multiple search engines. All of them that suggest that they are gathering stuff from across the Web but may not be doing that.