India allowed Facebook Inc. to start operating its WhatsApp payments service in the world’s biggest open technology market.

WhatsApp pay can go live using the homegrown, multibank Unified Payments Interface, the National Payments Corporation of India said in a statement Thursday. The U.S. firm can gradually expand its UPI base starting with as many as 20 million users. Facebook has been testing WhatsApp payments in India for years, but regulatory hurdles have kept the app’s pilot project to a very small number of users.

India’s payments market is crowded with domestic pioneer Paytm, Alphabet Inc.’s Google Pay, Walmart Inc.’s PhonePe, Amazon.com Inc.’s Amazon Pay and dozens of other startups. Yet, given its enormous user base of more than 400 million, WhatsApp has the potential to compete with the leaders and reshape digital payments in India — a market slated to grow to $1 trillion by 2023.

Unlike some other rivals, WhatsApp will be able to add customers for its payments service organically because of the popularity of its messaging app in the country. India is WhatsApp’s largest market, and an important country for Facebook, which is looking for areas to add new users as more lucrative markets like the U.S. and Europe become saturated.

Read more: Big Tech’s India Plans Can’t Seem to Bypass Asia’s Richest Man

Facebook earlier this year purchased a 9.99% stake in Jio Platforms, the digital services firm owned by Mukesh Ambani, India’s richest man, and Facebook has been open about its ambition to build a large commerce business in India with WhatsApp messaging at the center. Some analysts have said that Facebook’s partnership with Ambani would help the company navigate the country’s regulatory environment. Indian regulators have blocked some of Facebook’s previous business efforts in India, including a program that offered free internet access to a limited number of websites.

WhatsApp has added commerce features in recent years, including product catalogs and shops, so that small businesses can promote and sell goods directly through the app. Payments is a key part of enabling those transactions. The company is also hoping to use these product catalogs as a jumping off point to bring small businesses to other Facebook services, like advertising and customer service tools.

Facebook executives foresee WhatsApp serving as a one-stop-shop for Indian small businesses, who can sell goods and interact with customers on a service that hundreds of millions of people in the country already use. They hope to replicate that plan in other markets, like Brazil, though efforts to bring payments there have also faced regulatory challenges.

A WhatsApp spokesman declined to comment.

—Suvashree Ghosh (Bloomberg)

—Kurt Wagner (Bloomberg)