0801 GMT September 16, 2019
Journalists, celebrities and politicians, including Chancellor Angela Merkel, are among those whose personal data was published online, BBC wrote.
The attacks took place throughout December, but did not become public knowledge until Friday.
It is unclear who was responsible.
The contacts, private chats and financial details of figures from every political party – except the far-right AfD – were posted on Twitter.
The Interior Ministry says there is no evidence that parliamentary or government systems have been compromised – and it is not clear whether the attack was the result of hacking – or someone with access leaking the data.
The Federal Office for Information Security (BSI) has come under fire after it emerged that they knew about the leak since December, while the Federal Crime Office was only notified on Friday, news agency Deutsche Presse-Agentur and newspaper Bild reported.
BSI president, Arne Schoenbohm, told broadcaster Phoenix that his team "had already held corresponding talks very early in December with certain members of parliament who were affected," and launched a "mobile incident response team."
However, Dietmar Bartsch, parliamentary head of the left-wing Die Linke party, called the secrecy "completely unacceptable" and asked if the office had "something to hide."
Meanwhile, lawmaker André Han said, "It makes me unbelievably cross that yet again I've found out about such things from the media – even though I'm a member of the parliamentary monitoring group... the federal government's duty to keep parliament informed still applies between Christmas and the New Year."
National and local political figures as well as some TV personalities had their details stolen. They included Merkel – whose email address and several letters to and from the chancellor appear to have been published, the main parliamentary groups, Greens leader Robert Habeck – who had private chats with family members and credit card details posted online – and journalists from public broadcasters ARD and ZDF.
The true extent of damage caused by the leak is not yet known although Justice Minister Katarina Barley said it was a "serious attack."
"The people behind this want to damage confidence in our democracy and institutions," she said.