German police arrested a suspect in the stabbing on Friday evening at Berlin's Holocaust memorial that seriously injured a man two days before a watershed national election.
Berlin's police department gave no details on the identity of the suspect or his possible motive.
"Our forces have detained a suspect in the vicinity of the crime scene," city police posted on X. "Investigations continue."
Video of the scene showed emergency vehicles and heavily armoured police lined along one side of the memorial site, a vast field of grey concrete pillars where the attack took place. The memorial is across the street from the US Embassy.
The victim "was so seriously injured that he had to be taken by the fire brigade to hospital for emergency treatment," police spokesperson Florian Nath said.
The monument, one of the German capital's most sacred sites, commemorates the six million Jews murdered by Adolf Hitler's Nazis during World War Two, one of the darkest episodes in human history and a continuing focus of German historical atonement.
An eyewitness told local broadcaster RBB24 that the two men had appeared to approach each other before the victim was suddenly stabbed. Specialist fire brigade officers were offering trauma counselling to the witnesses, police said.
The area surrounding the monument in the heart of the city, was sealed off.
The national election campaign, in which polls suggest a far-right party could come in second place for the first time in nine decades, has been marred by a series of high-profile attacks.
One of those was a stabbing in which two were killed, including a toddler that was blamed on an Afghan immigrant. That prompted a fraught debate on immigration and demands from the front-runner conservatives for the border to be closed.
In December, a Saudi man who had lived in Germany for years and whose posting history showed he sympathised with the far-right, rammed a Christmas market with a car, killing six, including a child, and injuring hundreds.
Earlier on Friday, an 18-year-old ethnic Chechen was arrested on suspicion of planning an attack on the Israeli Embassy in Berlin, Bild newspaper reported.
