A teenage taxi customer was slashed three times across the face after a fight broke out over an unpaid fare.
The 18-year-old had jumped into a white licensed Hackney Carriage after a night out in Bradford city centre.
After picking up the man from a rank at Norfolk Gardens by City Hall, the taxi driver drove him home where there was a disagreement about the price of the fare.
Detective Constable Andy Elener, of Bradford South Police, said: "There was an argument between the pair and the driver brought him back to the cab rank where this continued."
The teenager later told detectives that at some point he punched out at the driver and a fight developed.
Det Con Elener said: "At least one other man joined in the violence and it appears that some kind of craft knife was produced and the man was slashed on the face."
He suffered an inch-long wound above the eye and another on his jaw line.
The slashing blade made another four-inch cut from below his left eye to his chin.
"A female who was close by saw what was happening and intervened in the attack," said Det Con Elener.
"She then brought the man across to the police station and he received medical treatment."
Det Con Elener said although the taxi driver had not inflicted the knife wounds, police had still been unable to trace him.
"We would appeal for him to contact us so that we can get this matter sorted out," he said.
Inquiries are continuing into the incident in the early hours of Sunday April 21. Police say every effort will be made to find both the knifeman and the driver.
"This was a vicious attack which left a man with serious facial wounds," said Det Con Elener.
"We would appeal anyone who witnessed it, or anyone who has information, to contact us at Bradford South CID on (01274) 376459."
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