Eight people were injured after a car ploughed into pedestrians in the centre of the northern Italian city of Modena, police said.
Four people are in a serious condition, the city mayor Massimo Mezzetti told broadcaster RaiNews24.
He said a woman was pinned against a shop window and required the amputation of both legs.
Police said they arrested the driver, a man in his thirties, and there was no further danger.
The driver is an Italian national born in Bergamo and a resident of the Modena area, the mayor added.
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"It seems (the driver) deliberately drove onto the sidewalk, hitting several people and crashing into a shop window. He then got out of his car and brandished a knife," said Mr Mezzetti.
He tried to flee but was stopped by some of the citizens involved in the crash and then by police, and was taken to headquarters for questioning.
One man who attempted to stop the driver suffered minor injuries, the mayor added.
Authorities are working to determine whether the driver was under the influence of substances or acted deliberately, the mayor said.
Witnesses reported the man was holding a knife, but he did not manage to stab anyone, the mayor said.
Victims were taken to hospitals in Modena and Bologna, with some of the most critical cases taken by helicopter.
"I express my solidarity with those who were injured and their families," Prime Minister Giorgia Meloni said in a post on X.
"What happened in Modena is extremely serious."
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