The charge in the European Commission’s antitrust complaint against Google may be relatively easy to explain — the company tried to steer people to its own comparison-shopping service. But it won’t be easy to show that Google’s tactic actually hurt its rivals — or hurt consumers by limiting their choices.

Europe’s commissioner in charge of competition, Margrethe Vestager, last week accused Google of damaging competing businesses by making their websites hard to find when shoppers search for products. Because Google has more than 90 percent of the market for Internet searches in most European countries, Ms. Vestager says the company violated European Union antitrust rules.

Image Margrethe Vestager, the European Union's competition commissioner. Credit... John Thys/Agence France-Presse — Getty Images

When users search for a product, say a piece of furniture, the results page shows a row of boxes at the top featuring various sellers offering the item, along with their prices. Google often places its service, called Google Shopping, ahead of links to other comparison-shopping services. Retailers pay Google if users click on their products on Google’s site. Some of Google’s critics compare how it displays results to Microsoft’s efforts to put the Internet Explorer Web browser on all PCs using its operating system. That practice led to a long antitrust battle. Now Microsoft, which paid European regulators $3 billion to settle antitrust charges, is one of Google’s biggest critics.