A 16-year-old boy was dragged from his bike by a gang of yobs who surrounded him with aluminium baseball bats.
The teenager was riding on Sunday afternoon when the mob approached him and threatened him with the weapons.
He was put in a headlock by one of the louts and then pulled from his saddle as his accomplices stood watch, carrying the bats.
One of the gang appeared to suggest he had a knife.
The violent incident happened on Tramway Road, in St Michael's-in-the-Hamlet, off Aigburth Road, at about 5pm.
The robbers escaped with the mountain bike, a Scott Aspect 950, which had been bought for the boy as a Christmas present.
It cost nearly £500.
Those responsible were said to be in their late teens.
The boy's mum described herself as "shocked" following the attack.
Her son, although uninjured, has been left badly shaken by the unprovoked assault.
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The 53-year-old told the ECHO: "I'm appalled.
"My children should be able to feel safe and enjoy the area in which they live.
"It was his own item in his possession, and he should be able to have it without being intimidated and threatened.
"It's horrific to know there are armed children in a relatively nice area of Liverpool."
The robbery started when two of the gang approached the 16-year-old, causing him to stop, before the rest of the robbers jumped out from a nearby bush, wielding the metal bats.
Officers from Merseyside Police responded to a report of the incident shortly after it happened and statements have been provided to the force.
Community patrols are to be stepped up in the area, the family was told.
The mum-of-two added: "This sort of thing shouldn't be happening, the community needs to come together to eradicate this."
One of the yobs had a balaclava on his face, while the others pulled their coats and hoods over their heads to try and hide their identity.
The gang tried to demand money and mobile phones, but they were told the boy didn't have any other possessions.
They rode and and ran off in the direction of Aigburth Road, towards Sefton Park.
CCTV has been requested from a nearby primary school as it is hoped that could help identify the culprits.
Merseyside Police are investigating.
Anyone who can help should contact police using @MerPolCC on Twitter or @CrimestoppersUK, or ring Crimestoppers anonymously on 0800 555 111.
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