Police said that a "firearm was used against the attacker," injuring the suspect. They said there was no further danger to the public.
"First responders, emergency services and a rescue helicopter have been deployed," they said.
The attack reportedly took place during a right-wing demonstration on one of Mannheim's central squares. Videos from the scene show civilians trying to subdue the attacker and him stabbing a police officer. A different policeman eventually shoots the suspect while the assailant is grappling with another man, the DW reported.
In an online statement, police said they could not immediately provide details on the number of the victims or the severity of the injuries.
However, local newspaper Mannheimer Morgen cited a police spokesman as saying that one of the officers on the scene sustained severe injuries. Later, the DPA news agency said the officer was undergoing surgery.