MUMBAI, India — Amazon’s founder and chief executive, Jeff Bezos, is visiting India this week for the first time in over five years.

Instead of garlands, India’s government is welcoming him with a new antitrust case.

The Competition Commission of India, the country’s antitrust regulator, opened a formal investigation on Monday into the practices of Amazon and Flipkart, the Indian e-commerce giant mostly owned by Walmart.

The inquiry was prompted by complaints from an association of small traders, after several rounds of regulations failed to curb the market power of the two e-commerce platforms, particularly in the online sales of mobile phones. Indian merchants have lobbied Prime Minister Narendra Modi to take tougher action against the companies.

India requires foreign-owned e-commerce firms to be neutral marketplaces, much like eBay, to protect local retailers and distributors from deep-pocketed competition. In the United States, Amazon both operates a marketplace and sells many products — including diapers, batteries and books — like a traditional retailer, buying them wholesale and then reselling them to consumers. Under Indian law, the site is supposed to rely on independent sellers who post their products on Amazon.