Yianni, 15, was spending the weekend with his father when he decided to go to the shopping centre for food and free wi-fi on Sunday morning.

At 12.30pm he was waiting at the bus stop when three teenage boys asked him for a cigarette. When Yianni replied that he did not smoke, the trio walked away, before returning a few minutes later.

"One started to touch my phone, and I asked: 'what do you guys want from me?'" Yianni told reporters at a police press conference on Tuesday.

The three boys then surrounded Yianni and began punching him.

"I yelled for help ... I looked at [bystanders] but they just kept on walking," Yianni said.

"I remember being punched and then I woke up with blood all over the floor and people telling me I was going to be all right."

Senior Constable Mark Vetter said the assault took "10 seconds, max" and the offenders made off with Yianni's phone.

"It was a pointless, opportunistic crime, and I think they just sensed the vulnerability of the young victim," he said.

"Unfortunately, the assault didn't get captured, because where it happened, it's a black spot. But there is footage of them running away towards Midlothian Street in Malvern East."

Yianni, a year 9 student at Oakleigh Grammar, has taken the week off school to recover from his injuries, which include a deep cut above his eye, a lump on the back of his head, two dislodged teeth, and emotional scarring.

Sia, Yianni's mother, was having lunch at Chadstone with her husband when she received a call from Malvern police officers, telling her Yianni had been attacked.

"I was just in shock, and Yianni was helpless and in pain and bleeding," she said.

Sia said she and Yianni's father had spent more than $6000 on braces for their son, which came off in May. On the Saturday before the attack Yianni had been given the all clear from his orthodontist.

"We'd just spent thousands on the perfect smile for Yianni ... and then this happened," she said.

"His two bottom teeth were knocked out from the bone, and [specialists] told us it has stopped the blood circulation."

She condemned the behaviour of the teenage attackers, saying she hoped they would be "punished and brought to justice ... to stop it happening to others".

"It's completely unacceptable to behave in such a violent way, and to approach innocent people who are just minding their own business," she said.

"I have to block [the anger] so that I can be strong for Yianni, but there come times where all I can see is Yianni being knocked and punched and not being able to defend himself. Never could I have imagined seeing him like I did in that ambulance,"

Police are asking witnesses who were in the area at the time to come forward.

Investigators have released images of three people they wish to speak to.

Anyone with information is asked to contact Crime Stoppers on 1800 333 000.