Data from celebrities and journalists also appear to have been leaked.
Said German Justice Minister Katarina Barley to BBC News, "The people behind this want to damage confidence in our democracy and institutions".
Beyond Germany, the hacking adds to concerns about the security of European parliamentary elections in May, which many officials fear are vulnerable to digital interference and disinformation campaigns by hackers or state-backed groups.
The information was leaked from a now-suspended twitter account @_0rbit operated from Hamburg, and Mr Seehofer said authorities were working to find the perpetrator.
Around 50 of the almost 1,000 cases of leaked data from German politicians are considered severe, meaning that personal data, photos and correspondence were made public, security sources told German Press Agency (DPA).
German cyber-security sleuths said yesterday they were aware of suspicious activity targeting a lawmaker's private e-mail and social media accounts last month, but could not make the connection with a mass intrusion targeting hundreds of politicians until the legislators' data was published on a Twitter account.
All major German parties except for the far-right AfD are affected.
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The Federal Office for Information Security called a crisis meeting on Friday to coordinate with the country's domestic and foreign intelligence agencies in investigating the leaks.
The daily Bild and public broadcaster RBB first reported the leak.
According to a preliminary investigation, "no sensitive information or data" had been leaked from Merkel's office, Fietz said.
The German news agency dpa reported that the information included a fax number and email address belonging to Merkel and several letters to and from the chancellor.
Suspicion will be directed at Russian state-sponsored hackers.
A German YouTube star whose Twitter account, @unge, was hijacked last week to disseminate links to the stolen material, said Saturday it appeared human error was to blame in his case.
"This hack clearly isn't about extortion or financially-motivated. This is about attempting to destabilise Germany society", he told the BBC.