The moment teen was stabbed in the heart at shopping centre

THE moments after a 17-year-old suffered a stab wound to his heart, leaving him in critical condition, have been captured on camera.

The daylight attack, around 2.20pm, outside a major Sydney shopping centre left the teenager in a critical condition and losing a lot of blood by the time paramedics arrived.

Police had described the wound as being to his "upper torso".

Footage of the alleged attacker was captured on Snapchat. Picture: Zaky Mallah

After the stabbing, the teen made it to a medical centre within the Westpoint Shopping Centre in Blacktown.

Surgeons and operating theatres were put on standby at Westmead Hospital as the young man began losing consciousness.

Police are now investigating the attack. Picture: James Feng / Twitter

Footage of the attack was taken by Zaky Mallah who later posted it to Snapchat.

"I saw this young lad, with a knife full of blood," he told Nine News.

"The bloke just stabbed someone and he was running away from the bloke that he stabbed, running towards me. I took out my phone to snap it and I thought he was going to come after me and stab me."

A second 17-year-old has now been arrested over the attack.

He faces charges of wounding a person with intent to cause grievous bodily harm and common assault.

He is due to face a Children's Court today.