UPDATE 1-German justice minister calls in Facebook over data scandal

(releads with Barley news conference, changes throughout)

BERLIN, March 22 (Reuters) - Germany's justice minister said on Thursday she had asked to speak to Facebook executives to find out whether the social media site's 30 million users in the country were affected by a scandal involving the handling of personal data.

The world's largest social media network is facing government scrutiny in Europe and the United States about a whistleblower's allegations that London-based consultancy Cambridge Analytica improperly accessed user information to build profiles on American voters that were later used to help elect U.S. President Donald Trump in 2016.

Katarina Barley said it must be possible for users of social media sites to specify whether they are happy for their data to be used in certain ways, rather than just giving them the option to tick "yes" or not be able to use the service.

"I demand clarification on whether German users and accounts are affected and what Facebook plans to do to prevent this from happening again," she told a news conference.

She said a day for the talks next week had not been set.

Germany has been a leading proponent of tougher regulation on social media. It passed a tough law to clamp down on online hate speech last year, and Facebook also faces a German anti-trust inquiry over the monetising of personal data.

Privacy rights are a particularly sensitive and emotional issue in Germany after decades of state surveillance carried out by the Nazi regime and later in Communist East Germany.

Facebook Chief Executive Mark Zuckerberg apologized on Wednesday for mistakes his company made in how it handled the data of 50 million of its users and promised tougher steps to restrict developers' access to such information.

The revelation has knocked nearly $50 billion off Facebook's stock market value in two days and hit the shares of Twitter and Snap over fears that a failure by big tech firms to protect personal data could deter advertisers and users and invite tougher regulation.

Barley said that the issue was best dealt with at a European, rather than a national, level. New European Union privacy rules take effect in May.

"We know that companies respect the rules when sanctions are particularly painful," she said. (Reporting by Joseph Nasr and Emma Thomasson Editing by Hugh Lawson)