The backlash against Facebook's "free mobile data" Internet.org scheme has spread across the globe.
A total of 67 digital rights groups - including i Freedom Uganda, Ecuador's Usuarios Digitales and Indonesia's ICT Watch - have signed a letter to Facebook's founder, Mark Zuckerberg, stating concerns about the initiative.
They say the project threatens freedom of expression, privacy and the principle of net neutrality.
Facebook continues to defend its offer.
"We are convinced that as more and more people gain access to the internet, they will see the benefits and want to use even more services," a spokesman told the BBC.
"We believe this so strongly that we have worked with operators to offer basic services to people at no charge, convinced that new users will quickly want to move beyond basic services and pay for more diverse, valuable services."
What is Internet.org?
Internet.org allows subscribers of partner mobile networks to use a limited number of online services without having to pay to make use of the data involved.
They include Wikipedia, the Facts for Life health site run by the United Nations Children's Fund, BBC News, Facebook, Accuweather and a selection of local news and sports results providers.
To access the facility, people must use special Android apps, Internet.org's website, Facebook's own Android app or the Opera Mini browser.
The web pages provided must be basic to minimise data use - high resolution photos, videos and voice chat facilities are not permitted.
Network operators participate because they believe users will pay for wider internet access once they have had a chance to try out the free content on offer.
Since 2014, the project has launched in Zambia, India, Colombia, Guatemala, Tanzania, Kenya, Ghana, the Philippines, Indonesia and Malawi.
Facebook says more than nine million people have used the scheme to date.
Until now, the most vocal opposition to Internet.org had come from India's tech community.
Local start-ups complained they risked being disadvantaged because they were not included, while several larger groups that were part of the scheme - including the media conglomerate Times Group and the travel booking site Cleartrip - pulled services, citing concerns about it failing to provide a "fair, level playing field".
The open letter from the 67 digital rights groups - which has been published on Facebook - makes clear that activists across the globe intend to challenge its expansion.
"It is our belief that Facebook is improperly defining net neutrality in public statements and building a walled garden in which the world's poorest people will only be able to access a limited set of insecure websites and services," it states.
"Further, we are deeply concerned that Internet.org has been misleadingly marketed as providing access to the full internet, when in fact it only provides access to a limited number of Internet-connected services that are approved by Facebook and local ISPs [internet service providers].
"In its present conception, Internet.org thereby violates the principles of net neutrality, threatening freedom of expression, equality of opportunity, security, privacy and innovation."
Earlier this month, Mr Zuckerberg sought to address concerns by inviting more organisations to apply to join Internet.org's platform.
But one of his requirements - that the websites should not include the HTTPS, TLS or SSL encryption technologies - caused fresh controversy.
In their letter, the campaigners suggest this will make users' web traffic "vulnerable to malicious attacks and government eavesdropping".
However, a Q&A published on Internet.org's site promises it will begin to support SSL and TLS "in the Internet.org Android app in the coming weeks".
And Mr Zuckerberg has posted a message to his Facebook page promising that support for HTTPS is also under development.
"We still need to do some work to make this work on all phones and browsers - so that's why our docs say it's not currently available - but we're going to make this happen soon," he wrote.
Even so, campaigners say they still have privacy concerns.
"Given the lack of statements to the contrary, it is likely Internet.org collects user data via apps and services," their letter states.
"There is a lack of transparency about how that data is used by Internet.org and its [telecommunications] partners."
The BBC understands that Internet.org does receive some information about which pages are visited, which it uses to ensure that consumed data is not charged for, as well as to monitor the popularity of different services.
However, there is no requirement for the partner services to share personal details about their users with Facebook.