Thursday, a lone gunman believed to be acting alone killed several people in a Jehovah's Witness church in the German city of Hamburg, authorities said, as they investigated the attack's motive.

The number of fatalities was withheld by the police, although the shooter was assumed to be among the deceased.

The Bild newspaper reported seven fatalities and eight injuries at the Kingdom Hall of Jehovah's Witnesses in the northern German city home to the country's largest port.

"According to the current state of affairs, we assume that there is one perpetrator," police said in a tweet.

"Police operations in the surrounding region are gradually being halted. The investigation into the crime's motivations continues."

Previously, Germany's DPA news agency, citing a correspondent on the site, said that inhabitants in the northern Alsterdorf sector of the city had received mobile phone alerts of a "life-threatening situation" and that streets were cordoned off.

Television images depicted scores of police cars and fire engines barricading streets and blanket-wrapped individuals being carried inside a bus by emergency service personnel.

"We heard shots," an unnamed witness said to the media. He stated that there were 12 continuous shots. Later we witnessed individuals being hauled away in black sacks.

Officers arrived at the scene just after 9 p.m. (2000 GMT) in response to a call from the police, where they discovered numerous persons critically hurt and some dead.

"Then they heard a shot from above, they went upstairs and found one further person," a police spokesperson said.

In the past few years, several shootings have shaken Germany. In February 2020, a gunman with alleged far-right ties killed nine people, including Turkish migrants, in the western town of Hanau before shooting himself and his mother.

On the Jewish holy day of Yom Kippur in October 2019, a gunman opened fire outside a synagogue in the eastern city of Halle. Two people were killed.

The mayor of Hamburg expressed outrage over the bloodshed on Thursday.

"I send my sincerest condolences to the victims' families. The forces are actively pursuing the offenders and investigating the incident's context, "Peter Tschentscher remarked on Twitter.