German police arrested 10 people in pre-dawn raids on Saturday on suspicion they were planning major attacks with stockpiled weapons and explosives, a prosecution spokesman said.
One of the 10 was confirmed as a Tajikistan national, but officials disclosed few other details about the suspects who were detained in the early hours in western and southern parts of the country.
Public broadcaster BR later reported that the 10 had been released but investigations were continuing.
Prosecutors said the suspects were arrested for allegedly plotting a "serious, state-endangering crime." Local media reported that they were sympathizers of the Islamic State militant group.
Germany has been the scene of several Islamist militant attacks in recent years, including a 2016 truck attack on a Berlin Christmas market that left 12 people dead.