BRUSSELS  Europe opened a formal antitrust investigation on Tuesday into accusations that Google had abused its dominance in online search, exposing the company’s zealously guarded technology to unwelcome scrutiny.

The investigation by the European Commission follows complaints from smaller Web businesses, which claim that Google downgraded their sites in its search results to weaken potential competitors for advertising. The commission said it would also look into whether Google might have given its Web services “preferential placement” in search results.

“Rigorous competition of all players, including smaller and innovative ones, must be preserved for the future,” Joaquín Almunia, the competition commissioner, told members of the European Parliament. He added, however, that the decision to upgrade the investigation from a preliminary inquiry that was started this year did not mean “that there is definitely a problem.”

Google’s dominance on the Internet has been a sore point in Europe, where it controls more than 80 percent of the online search market, compared with about 66 percent in the United States, according to comScore, a research firm.