Police say a 16-year-old Lalor boy and a 15-year-old Thomastown boy were walking along Spring Street, Thomastown, just before 1am on Wednesday when two cars pulled up beside them.

Police were told eight masked men then spilled out of the vehicles, and demanded cash and other personal items.

Leading investigator Detective Senior Constable Daniel Chrystie said the gang asked for "cash, money, mobile phones, wallets and whatever they [the boys] had on them".

The offenders then turned violent, attacking the boys with flick knives and knuckle dusters before tying them up in an illegally parked car on a bridge near Johnson Street.

Senior Detective Chrystie said the boys were walking from "one victim's house to the other victim's house" when the gang pulled up beside them.

He said the boys were tied up and placed in car about 100 metres from where the attack took place. The car is believed to be owned by a local resident.

Senior Detective Chrystie said the teens were in the car for a "maximum of 30 minutes" before an anonymous call to triple-0 alerted police.

"We're not sure who's called in, which is another reason why we're doing this [call-out to the public]," he said.

It is believed the violent gang was made up of men aged between 20 and 25, Senior Detective Chrystie said.

"[The] attack itself was already of a violent nature because of the fact that they were assaulted and tied up," he said.

"Any time weapons are produced it's a violent confrontation," Senior Detective Chrystie said.

"It's concerning that eight people would take on two young males for apparently no reason."

Senior Detective Chrystie said police were unsure as to why the teens were walking around on their own at 1am.

"It is school holidays, but it is a little concerning that people that age are out for no apparent reason," he told Fairfax Media.

"The fact that it was a cowardly attack on two young people is just unacceptable."

The 15-year-old boy was taken to the Northern Hospital for observation, but has since been released with no serious injuries, police said.

The 16-year-old from Lalor was taken in for treatment, and remains at hospital.

Police asked that anyone who may have witnessed the attack - including the anonymous triple-0 caller - to contact Crime Stoppers on 1800 333 000.