Teen critical after being stabbed in the heart at Sydney shopping centre

CCTV footage of the moment the teen is allegedly stabbed in Blacktown. Photo / Channel 9

The horrifying moment a teenager in Sydney's west was stabbed in the chest in daylight has been captured on camera.

The 17-year-old is in a critical condition in hospital after the brazen attack in Blacktown where he suffered a single stab wound to the heart.

He managed to reach the Westpoint Shopping Centre, where paramedics found the teenager in a critical condition and losing a lot of blood, NSW Ambulance chief inspector Brian Parsell told AAP.

The Careflight helicopter landed on the Prospect Highway so its emergency medical team could board the ambulance, which was rushing the teenager to Westmead Hospital, and stabilise him.

A witness told the Daily Telegraph the 17-year-old appeared to not notice he had been stabbed until someone else pointed out the blood. "The blood was flowing everywhere," Molly Nauer said.

Surgeons and operating theatres were put on standby as the teenager began losing consciousness. He was rushed into surgery as soon as he arrived.

"Anyone who has sustained a penetrating injury anywhere around the heart is a concern," Parsell said.

"Cardiac penetrating injuries are extremely life-threatening and time is of the essence."

Blacktown police set up a large crime scene and started investigating.

The 17-year-old was taken to hospital in a critical condition. Photo / Jammes Feng Twitter

NSW Police later arrested another 17-year-old at a Blacktown home in relation to the stabbing.

Video shared on social media shows a man brandishing a large knife and fleeing the scene.

Parsell praised the work and co-operation of the medical teams for giving the young man the best shot at survival.

The teenager remains in a critical condition, a hospital spokeswoman told AAP on Thursday evening.

The alleged attacker was charged with common assault and wounding a person with intent to cause grievous bodily harm.

He is due to face court today.

A family member said the 17-year-old - a member of a Seven Hills/Toongabbie gang called KNS - was retaliating after a cousin had been bashed earlier in the week.