The son of Germany’s former president Richard von Weizsäcker has been stabbed to death in Berlin.

Fritz von Weizsäcker, a prominent doctor and the head of a private Berlin clinic, was attacked while giving a medical lecture on Tuesday night.

Police said a 57-year-old man was arrested at the scene and was being questioned. They said the man was not known to police and was not believed to have been a patient at the clinic.

The suspect reportedly told police he wanted revenge on the family for the role the chemicals company Böhringer allegedly played in providing poisonous chemicals used in the Vietnam war. Richard von Weizsäcker was the company’s chief executive in the 1960s before he entered mainstream politics.

Witnesses said the attacker was sitting in the front row of the packed lecture room as Fritz von Weizsäcker addressed colleagues and patients. The man lunged at the doctor with a knife and stabbed him repeatedly.

An off-duty policeman in the audience attempted to stop him but was seriously injured in the process, police said. He was being treated in hospital. About 20 members of the audience then tackled the the man to the ground as he attempted to flee.

Von Weizsäcker, 59, had been giving a lecture on liver fat at the Schlosspark clinic, where he was head of the gastroenterology department.

He was the youngest of four children of Richard von Weizsäcker and his wife, Marianne. Richard von Weizsäcker served as Germany’s president between 1984 and 1994 and before that was Berlin mayor for three years. He was considered one of Germany’s most respected postwar politicians. He died in 2015.

Among the tributes paid to Fritz von Weizsäcker, his close friend Christian Lindner, the leader of the Free Democrats party, called him a “passionate doctor and a fine human”.

The attack is the latest in a series of high-profile knife attacks in Germany, including the near-fatal stabbing in 2015 of Cologne’s mayor, Henriette Reker, and the 2018 killing of a man in Chemnitz that sparked anti-immigrant riots. Experts say increasing numbers of people are carrying knives in Germany.

German crime statistics do not collate information specifically on knife crime, although authorities say they are working on producing nationwide statistics by 2022 amid growing concerns over the issue.

Böhringer Ingelheim has faced repeated criticism in the past for delivering 720 tons of trichlorophenoxyacetic acid to a New Zealand subsidiary of Dow Chemicals. The acid was a component for the fabrication of Agent Orange, the poisonous herbicide that was sprayed over wide areas of jungle by the US army during the Vietnam war.

Richard von Weizsäcker was the head administrator of the board of directors of Böhringer during this period, between 1962 and 1966.