German police have detained four men, one of whom they suspect of planning knife attacks at the Berlin half marathon, the newspaper Die Welt reports in its online edition.
Die Welt says the four suspects were linked to Anis Amri, a Tunisian man with Islamist militant ties who killed 12 people in an attack in Berlin in December 2016 when he hijacked a truck and drove it into a crowded marketplace.
Berlin police were not immediately available for comment.
In its unsourced report on Sunday, Die Welt says the main suspect was known to Amri and had planned to stab to death spectators and runners at the half marathon.
The suspect had in his possession two knives which had been especially sharpened for this purpose, the report said.
Amri's attack in Berlin in 2016 had prompted German lawmakers to call for tougher security measures. No major Islamist militant attack has been carried out in Germany since.
Earlier on Sunday, Interior Minister Horst Seehofer said the government would do everything possible to protect citizens, but added: "We have again experienced that ... absolute security is unfortunately not possible."
Mr Seehofer was speaking in Muenster, where a man drove a camper van into a group of people sitting outside a restaurant on Saturday, killing two of them before shooting himself dead.
Authorities have said the attacker was a 48-year-old German citizen with mental health problems and that there was no indication of an Islamist militant connection.