A teenager is in a critical condition after he was dragged off a bus, stabbed and left for dead in Joondalup on Saturday afternoon.

Emergency services were called to the scene around 4pm on Grand Boulevard near Boas Ave, forcing a stretch of the road to be closed.

Witnesses reported that a group of young men also smashed multiple windows on the bus during the attack.

Two other teenagers who were nearby fled the scene and sought refuge at a nearby home, reportedly shaken and terrified by the incident.

St John Ambulance crews arrived within five minutes of the call, dispatching multiple vehicles to manage the situation.

The stabbed boy was found unconscious and in a critical state.

He was transported under Priority 1 conditions to Joondalup Health Campus before being transferred to Royal Perth Hospital for further treatment.

He remains in a critical condition.

A second teenage boy was also taken to Joondalup Health Campus with minor injuries, he has since been discharged

Police were at the incident and unavailable for comment.

No arrests have been confirmed at this stage.

More to come.