An attacker on rollerblades stabbed three people on a busy street.
One of those injured was hurt as he attempted to stop the assailant in Wells Road, Knowle, Bristol, a witness said.
All three victims were treated by ambulance crews and taken to hospital.
Avon and Somerset Police said their injuries are not thought to be life-threatening and a man in his 30s was arrested in nearby Redcatch Road at about 15:10 GMT.
Witness Gary Stancer said he was making a phone call when he saw "a man on rollerblades come along the street and he came round then stabbed a man twice in the back".
He said his friend Rob was then stabbed as they both gave chase.
"It was an unbelievable event, surreal," said Mr Stancer. "This is Knowle - things like this don't happen here."
Michael Williams, who lives nearby, was on his way back from Broadwalk Shopping Centre at the bottom of the Red Lion hill when he saw the attacker give chase.
"All of a sudden I saw a man running in and out of cars in the traffic and then I saw a man chasing him on rollerblades with a knife in his hand," he said.
Mr Williams said he saw the attacker hide the knife in his back pocket after the victim escaped by crossing the road.
Police said they were keeping an "open mind about the motive" and an investigation is under way.