A young man's face was deeply slashed by a knifeman who attacked him from behind in Shepherd's Bush.
Borough police have issued an urgent witness appeal after the thug - who was with two or three others, all riding bicycles - threatened to stab his 27-year-old victim in the night-time attack.
The knifeman, who was only 5ft tall and aged about 20, then snatched his victim's laptop near Wendell Park in Wendell Road.
Case officer DC Alex Townsend, of Hammersmith police's robbery squad, said: "It was quite a deep cut across his face, which was treated in hospital.
"The victim was obviously very shaken up."
The 27-year-old Asian victim had been walking alone at 9pm, on Friday, January 30, when he noticed the group pass him on bikes.
His attacker grabbed hold of him from behind and sliced his face with a blade, before threatening to stab him.
When he took the bleeding man's laptop, the victim fled. The attacker then ran off with his accomplices towards Askew Road.
Becklow Road resident, mum-of_one Emily Phillips, who lives nearby, said: "This is a horrific attack.
"My boyfriend is a 6ft tall, strong man and I don't want him walking around alone late at night. This confirms my fears."
Borough commander detective chief superintendent Kevin Hurley, said the attack was carried out by a wicked and heartless person.
"The sentence I would like to see for this is a significant period in prison," he said, adding that the number of robbery offences are down by 21 per cent in the borough.
"People are a lot safer than they were," he said. "We put a lot of officers into protecting the public from these very unusual attacks."
His safety advice to the public is: "Do not worry or allow your quality of life to be affected by these very rare incidents.
"That said, don't leave attractive items on display whether it's on your person or in your car. There's no point advertising that you've got something to steal."
There is no description of the suspect, but the victim was wearing a black leather jacket, grey tank top, red shirt, black trousers and black shoes.
DC Alexander Townsend has asked anyone who might have seen or heard anything that day to contact him on 020 8246 2678.