MUMBAI, India - Amid fierce opposition in one of its most important markets, Facebook has launched an aggressive campaign in India for an app that gives users access to a small number of Internet services for free, arguing it could help lift millions out of poverty.

The California social media company has blanketed Indian cities with billboards and taken out two-page newspaper ads to tout Free Basics, which Indian regulators suspended last week after opponents contended it violated Internet neutrality, the principle that Internet providers allow equal access to all online content.

In an op-ed piece published Monday in the Times of India, the country's largest English-language newspaper, Facebook founder Mark Zuckerberg wrote that critics have mischaracterized Free Basics, which he said serves as a bridge to the Internet for millions who have never been online.

"If we accept that everyone deserves access to the Internet, then we must surely support free basic Internet services," Zuckerberg wrote. He compared the service to libraries, education, and health care - basic public goods that everyone expects to be available.

Libraries "don't contain every book, but they still provide a world of good," Zuckerberg wrote.

The struggle over Free Basics has evolved into a surprisingly bitter contest between Zuckerberg's goal to spread Internet access across the globe - while recruiting millions more Facebook users - and a diverse collection of Indian activists and entrepreneurs who accuse the company of creating a second-tier Internet service for people who can't afford to pay for data.

It also shows how India has become a battleground for tech giants, including Google and Microsoft, which are grappling for advantage in a country that will soon surpass the United States and claim the world's second-highest number of Internet users - with two-thirds of the population yet to be reached.

Free Basics is a preselected suite of Internet services for health care, education, weather, jobs, and communication - including, of course, Facebook - that the company has rolled out in more than 30 emerging markets, including India, in partnership with local telecom companies.

Users get those services - which vary by country - for free but must pay for access to other sites and apps outside of Free Basics. That has led to allegations that it hinders consumer choice and violates the net neutrality principle, which requires that Internet providers don't prioritize traffic to certain websites.

Critics say that by controlling which services are part of Free Basics, Facebook will determine the content that the poorest Internet users get to see. Mahesh Murthy, a Mumbai venture capitalist and net neutrality activist, has described it as "digital apartheid."