Facebook's Internet.org is facing growing opposition as digital groups claim that the free internet project violates principles of "net neutrality."

A letter addressed to Mark Zuckerberg, the company's chief executive was signed by 67 digital rights groups that includes i Freedom Uganda, Ecuador's Usuarios Digitales and Indonesia's ICT Watch. It states that Internet.org threatens freedom of expression, equality of opportunity, security, privacy and innovation, according to Telegraph.

"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," the letter 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."

The groups also stressed that Internet.org is misleadingly marketed since it is merely providing access to a limited number of Internet-connected services approved by Facebook and local ISPs and not to the full internet.

Facebook has responded to the letter saying that it shares a common vision with its critics. A spokesperson for the company said in a statement: "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... 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."

Internet.org was founded by Facebook in 2013 to make the internet accessible to more people by providing them access to a range of free basic services.