An "extreme danger" alarm was sounded in Germany's Hamburg and the police asked the residents to stay indoors as shots were fired upon a religious gathering in a church on Thursday.
At least seven persons were killed and eight others seriously injured in the attack on a weekly Bible study meeting at a Jehovah's Witness centre, local reports said.
Jehovah's Witnesses is a US Christian movement that preaches non-violence and practise door-to-door evangelism.
A gunman is believed to be among those killed, police said, without releasing any casualty figure.
The police used a catastrophe warning app to send out the danger alarm while nearby buildings were cordoned off.
We found a lifeless person in a community center who we believe could be a perpetrator, Hamburg Police tweeted, adding that searches are still on for the involvement of other attackers.
(With inputs from AFP and Reuters)