Four people were severely wounded in a stabbing attack at a gym, police have confirmed.
No arrests were announced, and police said they were seeking one suspect in the western German city of Duisburg.
The motive of the attack was not immediately clear.
The incident occurred in the old city of Duisburg and police asked residents to stay at home and avoid the area.
They said a large number of officers were at the scene of the attack, right across from the city hall.
Three people were in life-threatening condition, and one person was less severely injured, Duisburg police tweeted.
All four injured people are German citizen, police tweeted without giving any further details on their identities.
"According to current information, one person has injured other people with an object" at a gym, they tweeted earlier.
Police said that a "stabbing or cutting weapon" was used in the attack, but did not give any further details.
The attacker appeared to still be on the loose, German news agency dpa reported.
"There has been no arrest," police told dpa, adding that a hunt was under way for the attacker. Investigators also said it was too early to talk about a possible motive behind the attack.
Several victims were taken to a hospital, while investigators searched the gym, dpa reported.
Several witnesses to the attack, some still with their sports bags, were taken to a nearby restaurant so police could question them.
Helicopters were hovering above the gym and psychologists were brought in to help shocked witnesses.
Police said they received the first emergency calls at about 5.40pm local time.
Duisburg has about 500,000 residents and is located at the junction of the Rhine and Ruhr rivers. It used to be famous for its coal mining and steel production, but has in recent decades been plagued by high unemployment and poverty.
The president of the German parliament, Baerbel Bas, who is from Duisburg, expressed shock over the attack.
"Terrible. My thoughts are with the injured," Bas tweeted. "I hope that the emergency services will soon be able to clear up the threatening situation."
Several hours after the attack, the operator of the gym said it deeply regretted the incident.
"We deeply regret the incident at our club in Duisburg and hope that the victims will recover quickly," John Reed Fitness wrote in an email to dpa.
The gym chain, which has branches in several cities in Germany and also other countries, said it was supporting the police investigation and that "the safety and health of our members and our employees are our top priority."
Last month, a gunman stormed a service at his former Jehovah's Witness hall in the northern Germany city of Hamburg, killing six people before taking his own life after police arrived.
