A shooting at a Jehovah's Witness center in the German city of Hamburg has left eight people dead, including the suspected gunman, police said, as the motive for the attack remained unclear.

Several more people were injured in the attack, "some seriously," on Thursday evening at the Kingdom Hall building in the port city of Hamburg, where Jehovah's Witnesses members were attending a religious service.

The 35-year-old gunman, who shot dead seven people before turning the gun on himself was an ex-member of the religious group, police said.

Police identified the gunman as Philipp F., from Memmingen in Bavaria. 

"Philipp F. was a former member" of the Jehovah's Witnesses, according to Ralf Peter Anders, the Head of the Hamburg public prosecutor's office, adding that the attacker left the community about 18 months ago "but apparently not on good terms."

He also said, there was "no indication of a terrorist background" to the attack, yet forensic investigators are looking into a cell phone and laptop found at the perpetrator's house for evidence of a potential motive.

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German police had received an anonymous tip-off about the gunman prior to the attack but did not have enough grounds to take action at the time, officials said at a briefing in Hamburg.

German Chancellor Olaf Scholz called the attack a "brutal act of violence."

"Last night there was a terrible incident in my home town of Hamburg," said Scholz. "Several people have become victims of a brutal act of violence. It is feared that further victims will succumb to their severe injuries. We are stunned by this violence."

The chancellor went on to say his thoughts are with the victims and their families in these difficult hours. The Jehovah's Witnesses association in Germany said it was "deeply saddened by the horrific attack on its members."

"Hamburg has never seen a mass shooting like this," the state interior minister said. "There could have been more victims if the police hadn't intervened so quickly."

What do we know? by Natalie Carney in Germany

Citing one unnamed source, Hamburg police said, Philipp F. supposedly harbored anger towards religious followers especially towards Jehovah's Witnesses and his former employer.

Officials said the gunman, who they believe to have acted alone, had obtained the weapon legally. The victims included four men and two women, all German nationals, aged between 33 and 60 years. One woman was also pregnant with a 28 weeks old foetus.

In addition, six women and two men aged between 23 and 46 were injured. At least four of them sustained life-threatening injuries, some with multiple gunshot wounds.

The first emergency calls were made around 9.15 p.m. local time (20:15 GMT) after shots rang out at the building in the city's northern district of Gross Borstel.

"At the moment there is no reliable information on the motive of the crime," police said, urging people not to speculate.

An alarm for "extreme danger" in the area was sounded using a catastrophe warning app, but Germany's Federal Office for Civil Protection lifted it shortly after 3 a.m. local time.

The port city's mayor, Peter Tschentscher, expressed shock at the shooting on Twitter. 

Police have asked witnesses to come forward and upload any pictures or videos they may have to a special website. Interior minister Nancy Faeser said investigators were "working flat-out to determine the background" to the attack.

The first officers at the scene found several lifeless bodies and seriously wounded people, police said. Officers heard a shot in the "upper part of the building" before finding a body in the area where it rang out, police said.

Hamburger Abendblatt reported that 17 unhurt people, who had been at the event, were being attended to by the fire brigade, in a tweet.

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