A 17-year-old was walking along a Napier street listening to music yesterday when he was suddenly punched in the back of the head.

As the teenager picked himself up off the ground a shirtless young man was "shaping up to attack again while yelling obscenities at him," a police spokesman said.

He ran home and called the police. Police want to speak to a group of five young men who were walking nearby when the attack occurred on the corner of York Ave and Lancaster about 5.45pm yesterday.

The victim suffered a sore head, scratches and a cut lip. He was very shaken and did not know his attacker. The alleged offender is described as a 17-year-old man, with brown skin and medium hair who was not wearing a shirt.

A police spokesman said the random and cowardly nature of this attack was concerning. No other incidents of this type had been recently reported.

It comes a day after a 12-year-old boy was assaulted by a man who took exception to him calling out "hello".

The man, believed to be between 40 and 50 allegedly grabbed the boy by the throat, punched him and put him in a headlock, causing him to fall down a stock bank. The man is then alleged to have chased the boy, kicked him and put him back in to a headlock. He then let him go and rode off.

The boys ran to the nearby carpark and told people what happened.

Police were still looking for the man who was wearing black pants and a grey top and riding a mountain bike with a large black rear mudguard

People with information on either attack can contact Hawke's Bay Police on 06 8310700 or anonymously via Crime Stoppers on 0800 555 111.