No arrests have been announced and police said they were searching for a suspect. The motive for the attack was not immediately clear.
The incident occurred in Duisburg’s Old Town and police asked residents to stay home and avoid the area. Police said a large number of officers were on duty at the scene, directly opposite the town hall.
According to police, the first emergency calls came in at around 5:40 pm local time.
Three people’s lives were in danger, one person was less seriously injured, Duisburg police tweeted. All four injured were German nationals, police tweeted, without giving further details on their identities.
Police said a “stabbing or cutting weapon” was used in the attack, but gave no further details.
The attacker still appears to be at large, German news agency dpa reported.
Several victims were taken to hospital while investigators searched the gym, dpa reported. Several witnesses to the attack, some of whom still had their gym bags with them, were taken to a nearby restaurant so police could question them.
Duisburg has about 500,000 inhabitants and is located at the junction of the Rhine and Ruhr rivers. The city used to be known for its coal mining and steel production, but in recent decades it has been plagued by high unemployment and poverty.
A few hours after the attack, the gym’s operator said he deeply regretted the incident.
The gym chain, which has branches in several cities in Germany as well as other countries, said it supported the police investigation and that “the safety and health of our members and our staff are our top priority”.
Last month, a gunman stormed a church service at its former Jehovah’s Witness hall in the northern German city of Hamburg, killing six people before taking his own life after police arrived.
