The son of a former German president has been stabbed to death while delivering a lecture at a clinic in Berlin.
Fritz von Weizsäcker, 59, was attacked on Tuesday evening by an audience member wielding a knife at the Schlosspark Clinic, where he was a head physician.
A 33-year-old off-duty police officer who intervened was also seriously injured in the attack, Berlin police said in a statement, and was taken to hospital for treatment for non-life threatening injuries.
His intervention, along with several audience members, eventually led to the arrest of the 57-year-old attacker, the statement added.
The attacker was not previously known to police and was not believed to be a patient at the clinic. He is due to appear in court on Wednesday.
Weizsäcker, who was an expert in gastroenterology, hepatology, oncology and infectiology, was delivering a lecture to around 20 people about liver fats when he was killed.
He is also the son of the late German president Richard von Weizsäcker, who served in the role between 1984 and 1994, and died in 2015.
In response to the attack, the spokesman for German Chancellor Angela Merkel said it was a "horrible blow", and expressed condolences to the family.
"We don't know much about what happened here in Berlin last night,'' he said, adding: "It's a horrible blow to the von Weizsäcker family and the chancellor's condolences, and certainly also those of all the members of the government, go to the widow, to the entire family.''
Germany's Free Democratic Party leader Christian Lindner said he was "stunned" by the death of his friend.
Describing him as "a passionate doctor and a fine person", he wrote on Twitter: "Recently, he was at my home for a barbecue. I am stunned and must share my grief.
"Once again, one asks oneself in which world we live."
The victim's sister Beatrice also shared a post to social media, which simply said: "Take care of my brother" next to a picture of a Christian cross.