Detectives are today questioning a teenage suspect after a fellow schoolboy was stabbed on a packed bus outside a school in Highbury.

The victim, 16, was left fighting for his life after being airlifted to hospital by a helicopter which landed in the grounds of Highbury Grove School on Tuesday afternoon.

A 15-year-old suspect is being held by officers after he was arrested on suspicion of attempted murder last night. Police are appealing for witnesses for the after-school attack at about 3.20pm which it is thought came following an altercation.

Onlookers spoke of their horror as they watched paramedics give the teenager life-saving treatment on the single deck 393 bus, which performed an emergency stop near the school in Highbury New Park.

Lisa Mitchell-Smith, who lives in a ground floor flat of the nearby Spring Gardens Estate, said the victim was taken by stretcher across the road from the bus to the air ambulance.

She said: “There were about six policemen covering him with sheets – but you could see blood dripping everywhere.”

Mrs Mitchell-Smith added how a youth ran past her kitchen window down an alleyway just moments after the bus stopped.

Another estate resident, whose flat overlooks where the bus stopped, said police told him the victim had a low chance of survival.

A Metropolitan police spokesman said the schoolboy is in a “critical but stable condition” in an east London hospital. A cordon remained at the scene until about 10.30pm.

Just hours before the attack the school had been visited by Andrew Lloyd Webber who was supporting the Music in Secondary Schools Trust.

Joseph Smith, who also lives on the Spring Gardens Estate, said: “All I saw was them reviving somebody in the bus, giving them CPR for about 30 minutes, pumping up and down on their chest.

“Police were working on him before paramedics got there.

“They were working on him for ages. I was thinking maybe he died the way they were working on him. It looked like he was losing his fight. It was shocking, just shocking.”

Police scoured the estate, including gardens and forecourts, in case a weapon had been dumped nearby. Highbury Grove after-school clubs were postponed.

Raffaele Buttice, whose flat overlooks the scene at the junction with Holmcote Gardens, said: “I could see three people on the floor of the bus doing heart resuscitation.

“The bus was packed, but they were being let off gradually after the driver locked the doors I think.”

A police spokesman said officers were called at 3.20pm to the scene at the junction Holmcote Gardens. Call Police on 101 with information or Crimestoppers anonymously on 0800 555 111.