Three people were killed and four severely wounded in a knife attack at a festival in the western German city of Solingen on Friday, according to a spokeswoman for the police in nearby Duesseldorf.
Police have launched a "major operation" to find the suspect, who is on the run, the spokeswoman said, adding that a "wide area" had been cordoned off.
Police were at the scene with helicopters and emergency vehicles, and had asked the public to avoid the area, according to the broadcaster ZDF.
The festival was part of a series of events to celebrate the city's 650th birthday.
In a statement posted online, Tim-Oliver Kurzbach, the mayor of Solingen, said the whole city was in "shock, horror and great grief".
"We all wanted to celebrate our town's anniversary together and now we have to mourn the dead and injured," he said.
Kurzbach thanked the emergency services for their work at the scene and expressed sympathy with those who had witnessed the attack.
"It tears my heart apart that there was an attack on our city. I have tears in my eyes when I think of those we have lost. I pray for all those who are still fighting for their lives," he said.
Solingen is a city of some 150,000 people located an equal distance from Duesseldorf and Cologne.
People had gathered in the town on Friday evening for the first day of a three-day "Festival of Diversity", according to the event's website.
The festival was set to feature music, street theatre, variety shows and comedians in the city centre and several other areas, it said.
Up to 75,000 visitors were expected to attend over the three days.
The Solinger Tageblatt newspaper reported that one of the co-organisers of the festival had come on stage to cancel the event.
The crowd were also asked to leave the city centre, it said.
Following the announcement, thousands of attendees cleared the area, the paper reported, with a journalist at the scene describing the atmosphere as "ghostly".
"People left the scene in shock, but calmly," Philipp Mueller, one of the organisers, told the newspaper.
A witness who spoke to the Tageblatt said he was a few metres from the attack, not far from the stage, and "understood from the expression on the singer's face that something was wrong".
"And then, a metre away from me, a person fell," said the man, Lars Breitzke, who at first thought it was someone who was drunk.
But when he turned around, he saw other people lying on the ground and several pools of blood, he added.
Germany has seen a series of knife attacks over the past 12 months, with Interior Minister Nancy Faeser promising to crack down on knife crime.
A police officer was killed and five people were wounded in a knife attack at a far-right rally in the city of Mannheim in late May.
