* Germany investigating far-reaching breach of data security
* IT worker helping police with investigations
* Greens leader cancels Facebook, Twitter accounts
By Andrea Shalal and Hans-Edzard Busemann
BERLIN, Jan 7 (Reuters) - German government officials called on Monday for a tightening of data security laws after a far-reaching breach exposed the documents of hundreds of politicians, including Chancellor Angela Merkel.
The leader of the opposition Greens party said he had cancelled his Twitter and Facebook accounts following the data breach, one of the largest ever in privacy-conscious Germany.
Stephan Mayer, state secretary in the Interior Ministry, said authorities were working around the clock to investigate the breach, and would provide more details around mid-week.
Mayer reiterated that government networks had not been breached, but said the ministry was working to tighten the cyber security of critical infrastructure and would also beef up the federal cyber security centre leading the current investigation.
"One bit of positive news is that government networks are apparently not affected by this or these hacker attacks. But it's clear that we as the federal government ... must do more to improve cyber security," he said.
German police on Sunday searched the apartment of Jan Schuerlein, a 19-year-old IT worker in the southern city of Heilbronn.
Schuerlein said on Twitter he was being treated as a witness in the investigation given his contact with "0rbit", the holder of a Twitter account that was used to publish the data but has since been shut down.
Schuerlein said he was cooperating with police, according to broadcaster rbb, which first reported the police search. Little is known about the user "0rbit".
A spokesman for state prosecutors declined to comment on Sunday's raid, but said the investigation was continuing.
Justice Minister Katarina Barley and Germany's digital coordinator, Dorothea Baer, also called on Monday for tougher social media controls following the data breach, which began in December but initially went largely unnoticed.
"Such an attack must be used as a reason to very carefully examine if everything has been done to achieve the best possible security of data," Baer told the Handelsblatt newspaper.
"It is legitimate to examine whether software manufacturers and platforms must be required to do more to ensure data security," she told the paper, without providing any details.
Robert Habeck, leader of the Greens, said he had cancelled his Twitter account after he spoke disparagingly about the eastern German state of Thuringia in a video tweet. He said the medium caused him to be "more aggressive" than he would otherwise be.
Habeck said he was also deleting his Facebook account because it was the main tool used to spread private data published as a result of the breach, including chats with his family members.
No comment was immediately available from Twitter. Facebook declined to comment on the data breach issue. (Reporting by Andrea Shalal and Hans-Edzard Busemann in Berlin, and Reuters TV in Cologne Editing by Gareth Jones)