The police, including tactical and bomb disposal units, were on the scene late Thursday, and the injured were rushed to local hospitals.
Local media said the incident happened at a Jehovah’s Witnesses centre at about 9pm local time (20:00 GMT) on Thursday night.
The Bild newspaper reported that eight people had been killed and eight others injured.
A police spokesman declined to say the number who had been killed and said the motive for the attack remained unclear.
Little was immediately known about the assailant, and the Hamburg police said early Friday that they believed there was just one. A police spokesman indicated that the attacker might be among the dead.
“All we know is that several people died here and that several people have been injured and have been brought to hospitals,” Holger Vehren, a Hamburg police spokesman, said.
There had been an event at the hall at the time of the shooting, and many abandoned winter jackets could be seen hanging in the foyer of the three-story building in local news footage.
The police were first called to the scene at 9:15 p.m. local time. When they arrived several minutes later, they found the dead and the wounded and heard a final shot on the top floor of the building, Mr. Vehren said. When police forces reached that floor, they found another dead body.
“Whether or not this was the perpetrator is not yet 100 percent clear, but at the moment we can assume it is,” Mr. Vehren said.
The police are still investigating a motive and whether the attacker was specifically targeting Jehovah’s Witnesses, a small religious community in Germany, where there are about 175,000 adherents across the country and nearly 900 dedicated Kingdom Halls.
Authorities issued a warning asking people to avoid the area and stay indoors.
The assault happened in Gross Borstel, a neighborhood between Hamburg’s downtown and the airport. The hall itself is on a busy, six-lane road, neighboring a gas station and other commercial properties.
The Hamburg police asked that social media users refrain from conjecture on the motive or chronology of the attack while forensic teams worked.
Peter Tschentscher, the mayor of Hamburg, promised that authorities would work quickly to secure the crime scene and reconstruct the assault.
“My deepest sympathy goes to the relatives of the victims,” he posted on Twitter.
