Police have raided dozens of properties in Germany and arrested a Tunisian man suspected of being a recruiter for Islamic State and rounding up supporters to carry out an attack.
Frankfurt prosecutors said 54 flats, business premises and mosques in the region were searched early on Wednesday.
They said the investigation was directed against 16 people in total, aged between 16 and 46.
The 36-year-old main suspect was arrested in Frankfurt. He is believed to have been a recruiter and smuggler for the IS group since August 2015.
Prosecutors say plans for a possible attack were at a very early stage and no concrete target had been chosen.