It comes after Ecuador cut off communications for Mr Assange, who has been living inside the country‘s London embassy for more than six years.
Baltasar Garzon, a lawyer for WikiLeaks, has arrived in Ecuador to launch the case, which is expected to be heard next week in a domestic court.
WikiLeaks claims Mr Assange‘s access to the outside world has been “summarily cut off” and says Ecuador has threatened to remove the protection he has had since being given political asylum.
Image: Mr Assange claims his 'fundamental rights' have been violated
The site said Ecuador‘s government has refused to allow a visit by Human Rights Watch general counsel Dinah PoKempner and prevented several meetings with Mr Assange‘s lawyers.
A statement said: “Ecuador‘s measures against Julian Assange have been widely condemned by the human rights community.”
He has been inside the building in Knightsbridge since June 2012 in order to avoid a European Arrest Warrant over a rape claim in Sweden.
This investigation has been dropped, but Mr Assange fears he could be extradited to the US over WikiLeaks‘ publishing of huge amounts of classified information.
Lawyers for Mr Assange have said they will also challenge the legality of the Ecuador government‘s “special protocol”, which make his political asylum contingent on “censoring” his freedom of opinion, speech and association.
That protocol requires journalists, lawyers and others meeting with Mr Assange to disclose details including social media usernames and the serial numbers of phones and tablets with Ecuador, which can than share the information “with other agencies”.
In the statement, Mr Assange‘s legal team claims the Ecuador embassy can seize his property or that of his visitors and, without a warrant, hand it over to UK authorities.
WikiLeaks said that US congressmen had written an open letter to Ecuador President Lenin Moreno stating that in order to advance “crucial matters… from economic co-operation to counternarcotics assistance, to the possible return of a USAID mission to Ecuador, we must first resolve a significant challenge created by your predecessor, Rafael Correa – the status of Julian Assange”.
The site believes Ecuador is under increasing pressure to hand Mr Assange over to UK authorities.
The lawsuit is said to centre around his isolation, the ban on visits, speech restrictions and a failure to live up to international obligations.
Last month, an leak of internal WikiLeaks files revealed Mr Assange .