Four boys tackled a man who had snatched a woman's mobile phone while wielding a meat cleaver.
Officers are looking for the boys, who are described as Asian and believed to be in their early to mid-teens, who took down the culprit a short time after he took a phone from a house in Falmouth Avenue, Bradford, at around 7pm on Tuesday.
They were all wearing dark clothing and one of them had a silver bicycle, police said.
It is thought they were alerted to the offence by the victim, who is believed to have chased the culprit from her house.
The boys confronted him in Prospect Road and managed to recover the phone.
Nobody was hurt.
Detective Inspector Ryan Bragg, of Bradford District CID, said: “These boys were very brave in confronting the suspect and we are keen to speak with them to see if they can assist our investigation.”
The culprit is thought to be Asian and is believed to have been older than the boys.
He is thought to be around 5ft 8ins tall and was wearing a black jacket and a scarf to cover his face.
Anyone who can identify the boys or who may have information about the offender is asked to contact Bradford District CID on 101, quoting crime reference 13170591918.
Information can also be reported anonymously to the independent charity Crimestoppers on 0800 555111.