Digital rights groups have accused Facebook's Internet.org project of violating net neutrality.

The social network's CEO, Mark Zuckerberg, wants to bring free internet to two-thirds of people across the world, citing benefits like better access to education, health information and jobs.

But an open letter signed by 67 separate organisations claims the project will create a two-tiered internet as well as risk the privacy of its users.

The letter points out that in its current form, Internet.org would only provide users with a handful of free sites and applications, which are approved by Facebook and the ISPs it works with on the scheme.

This approach promotes some web services over others, and drastically reduces the portion of the internet people are able to access, the letter states.

"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," the letter said.

To access the full internet, users would be required to pay for access a strategy the digital rights groups said "fails to acknowledge the economic reality for millions of people".

The current implementation of Internet.org also doesn't support HTTPS, the secure communication protocol, but Zuckerberg has stated that it will incorporate this in an update.