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 sympathisers 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.
Authorities across Europe are also on high alert for attacks from right-wing extremists after last week’s New Zealand mosque attack that left 50 dead.
Austrian police are investigating links between the attacker and the far-right Identitarian Movement, which opposes Muslim immigration.