Germany has been put on increased security alert through fears of a heightened threat from international terrorism.
The move followed new intelligence, said Thomas de Maiziere, the country's top security official.
Germans will see a visible response to the threat, including more police at airports and railway stations.
Mr de Maiziere urged calm, saying: "There is reason for worry, but there is no reason for hysteria," he said. "We will not allow international terror to limit us in our way of life and our culture of freedom."
Germany has no colour-coded or numbered-tier system for measuring security levels, comparable to those in the US, Britain or France and tends to be far more cautious in publicly discussing the degrees of threat, on grounds it could endanger efforts to prevent an attack.
Although Germany has escaped any major terrorist attacks like the Madrid train bombings of 2004 and the London transport attacks of 2005, at least two major plots have been thwarted or failed before they could be carried out.
"Besides the previously known findings, we now have additional, relevant indications of possible danger that security authorities agree justify our current assessment that we are now facing a new situation," Mr de Maiziere said.
He cited a tip from another unspecified country about a suspected attack planned for the end of November - though he did not say where it was supposed to take place.
That tip arrived after the interception of two bombs mailed from Yemen to the US - one of which went through a German airport before it was found in Britain. The Yemen plot points to "the adaptability and the persistence of terrorists in pursuing their aims," and also underlines "the reliability of some leads," Mr de Maiziere said.
He said security services had also gathered concrete intelligence of their own pointing to "sustained efforts" by Islamic extremist groups to plan attacks in Germany.