German police issued a warning on Saturday that there was an imminent threat of a violent Islamist attack on the northern port city of Bremen.
A federal authority has been receiving tip-offs since Friday evening about the activities of potential Islamist threats, the police said.
Police said they were adopting "security measures in the public arena" without going into detail.
Recently, police cancelled the pre-Lenten carnival in the city of Braunschweig, or Brunswick, 175 kilometers south-east of Bremen over a terrorist threat.
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