A 15-year-old was stabbed on Manchester's main shopping street.
The boy had been shopping with a 16-year-old friend
The teenager was confronted by his attacker after he and his friend saw the man involved in altercation in Market Street on Sunday.
The attacker saw him watching and chased him, but the boy fell over and the man stabbed him in the back.
The assailant was black, aged about 18 or 19, 5ft 10ins tall, medium build with short black curly hair and a prominent gold tooth.
The boy and his friend were near Debenhams, at the junction of High Street and Market Street when they saw two men acting aggressively towards a third man.
A police spokesman said: "As the boy was watching what was happening, one of the men turned on him and asked him what he was looking at.
"The man was carrying a knife and so the boy backed away across High Street.
"But the man followed him and the boy struck out with a carrier bag in self-defence. The 15-year-old then turned and tried to run away, but he tripped and fell to the floor.
"As he was on the floor, the man stabbed him in the back and slashed him across the face, before running off."
Market Street would have been very busy with shoppers around the time the assault happened, at about 1400 BST.
Police are asking witnesses to the attack to contact them.