Federal authorities had been receiving tip-offs since Friday evening about the activities of potential Islamists, the police said, without clarifying whether it was a terrorist threat.

People in Bremen were said to have remained relaxed despite the apparent threat. A Bundesliga match between Werder Bremen and Wolfsburg at the Weser stadium was scheduled to go ahead as planned on Sunday.

In the evening, police searched an Islamic cultural center near the railway station, refusing to give any details of the operation for the moment. Heavily armed police were sighted in the city throughout the day, protecting places of special significance such as Jewish community sites, the town hall and the marketplace.

A police spokesman revealed on Saturday evening that "some persons have been detained." There had been an official arrest as well, he added. Again, the spokesman did not give any details, merely referring to a police statement released earlier in the evening, in which the talk was of a "security net" which had been set up to cover the city center.

The statement also mentioned protective steps of a preventive nature which had been taken to enhance the security of the Jewish community of Bremen. The search at the home of a suspect and at the Islamistic Culture Center (IKZ) were in the course of a running investigation, the statement said.

ac/rc (dpa, Reuters)