Facebook has been given four weeks to reply to a class action over online privacy. 

A court in Vienna has told campaigners behind the lawsuit they have passed the first step in their battle against the social network. 

Max Schrems, the Austrian lawyer leading the Europe-V-Facebook challenge, revealed 25,000 users from outside the US and Canada have signed up to support the case and another 35,000 have also registered support. 

The privacy activist said the Vienna Regional Court confirmed the lawsuit has passed its initial test and Facebook has been ordered to respond. 

"The order is very likely on the way to Facebook. The first step in the legal procedure is hereby taken," Mr Schrems said.

Facebook has an initial four weeks to reply but can ask for an extension to double the timeframe. 

Mr Schrems said that the court will have the power to make a judgment in absence of a counter-statement from the social network. 

The campaign has been overwhelmed with support since it was launched at the start of the month, with 7,000 users a day from over 100 countries registering their backing at its height. 

Mr Schrems is claiming damages of €500 per supporter in the courts in Vienna for alleged data protection violations by Facebook, including over the US Prism spy programme. 

The action is being taken against the Irish subsidiary of the New York-listed web giant. 

Mr Schrems has been challenging the social network's use of data through his Europe-V-Facebook.org campaign and the Data Protection Commission in Ireland and has more than 20 active complaints of alleged data breaches filed with the watchdog. 

The class action claims Facebook Ireland is in breach of European law on users' data and it violates rights by tracking internet use on external sites, including the use of "like" buttons. 

It also attacks Facebook's analysis of users through what it calls "big data" systems. Mr Schrems claims the company supports the Prism surveillance programme, the US secret service's worldwide monitoring and data mining exposed by whistleblower Edward Snowden. 

An earlier landmark battle launched in Ireland to find out what Facebook tells US spy chiefs was referred to the European Court of Justice by a judge in Dublin last month. 

Facebook has more than 1.3 billion users. Its shares are trading on the Nasdaq at around $72, making it worth about $200 billion. 

The company declined to comment on the first stage of the class action lawsuit.