“Police and intelligence services will continue their intensive investigations and maintain the protective measures for the Jewish community,” city security senator Ulrich Mäurer said on Sunday.
Authorities had searched several homes over the weekend after receiving a tip that a man had been planning to buy submachine guns and pistols and sell them on further.
“These clues were so concrete that we could no longer exclude the possibility of an attack in Bremen,” Maurer said.
He added that there was a possibility that Islamists in the city could have bought the weapons.
Police arrested A 39-year-old man and one other over the course of the weekend and are being investigated for breaching weapons ownership laws.
The alarm in Bremen is the third one this year, after police forbade a January demonstration by anti-Islam movement Pegida in Dresden over fears of an attack and the cancelling of the carnival parade in Braunschweig.
Chairman of the German Police Union Rainer Wendt told the Passauer Neue Presse on Monday that “terror is no longer abstract, but very concrete,” adding that the public would have to get used to such incidents.
And Wolfgang Bosbach, president of the Bundestag (German parliament) Home Affairs committee, told the same newspaper that “after Dresden, Braunschweig and Bremen, it's more and more difficult to say that there's only an abstract danger.”