Germany is increasing security at airports and railway stations in light of concrete indications of terrorist attacks being planned for the end of this month.
Interior Minister Thomas de Maiziere says it follows a tip-off from another, unnamed country.
He said in Berlin that Germany has information on sustained efforts by Islamist extremists to carry out attacks.
Extra security will remain in place until further notice.
Two parcel bombs were intercepted en route from Yemen to the United States last month.
One of the bombs was despatched via the German city of Cologne, but was intercepted in Britain.
Mr de Maiziere said Germany would not allow international terrorism to constrict its way of life or liberal culture.
He said the federal police force has been ordered to step up checks at airports and train stations.
Mr de Maiziere said some of the new security measures would be clearly visible.
A month ago, Mr de Maiziere was dismissive of American and British warnings of terror attacks in France and Germany.