Notorious drug lord Tony Mokbel is in a critical condition in hospital after being stabbed at Victoria's maximum security Barwon Prison.

It is understood the 53-year-old gangland criminal suffered multiple stab wounds to the chest about 3.45pm on Monday and was left bleeding from the mouth, head and upper body.

Mokbel was one of two prisoners attacked on Monday afternoon.

The second victim, aged 31, was taken to hospital by ambulance in a serious condition.

Media reports allege Mokbel was set upon by two attackers moments before the prison was placed into lock-down.

It is alleged the second man stabbed was injured after coming to the drug kingpin's aid.

Tony Mokbel is treated by paramedics outside the prison. Nine News

A police source said it was believed the attack was related to a report in the Herald Sun on Sunday that suggested Mokbel was a "top enforcer" at the prison and that he had intervened in a prison stand­over scheme, disrupting an extortion racket being run by Pacific Islander inmates.

Mokbel was flown to the Royal Melbourne Hospital in a serious condition where he remains in intensive care.

He was still conscious when he arrived at the hospital and was seen shaking the hand of a paramedic who treated him as he was being wheeled in.

But his condition has deteriorated with a hospital spokeswoman saying Mokbel was listed as critical at 6.30pm.

Tony Mokbel shakes the hand of a paramedic at Royal Melbourne Hospital. Nine News

Aerial footage of the aftermath of the stabbing showed paramedics and police gathered around a shirtless Mokbel who was lying on stretcher in a pair of shorts.

Paramedics were seen holding a white sheet above Mokbel as he was treated in an attempt to shield him from the media.

It is understood paramedics treated Mokbel inside the prison before taking him to the waiting air ambulance outside.

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Tony Mokbel being stretchered out of Barwon Prison. Nine News

"The investigation is ongoing and more details will be provided when appropriate to do so," a police spokesman said.

Mokbel is serving a lengthy sentence for serious drug crimes. He was sentenced to 30 years' prison in 2012, to serve a minimum of 22 years.

However, he is one of a number of criminals whose conviction is now in doubt after it was revealed a gangland criminal barrister acted as a police informer and gave up information about her own clients.

Paramedics treat Tony Mokbel. Nine News

The explosive legal scandal has led to Premier Daniel Andrews announcing a royal commission into Victoria Police's mismanagement of some of the state's most high-profile criminal investigations.

The scandal, that stems from Melbourne's bloody gangland war, deepened last week when it was revealed the lawyer, known as Informer 3838 and Lawyer X, was in fact registered as a police informer 10 years earlier than Victoria Police had previously admitted.

Mokbel is known for skipping bail and fleeing to Greece before he was due to face trial for trafficking amphetamines in March 2006.

The drug baron spent about six months hiding at an associate's modest country property at Bonnie Doon in Victoria, before he was moved to Western Australia. He was then smuggled out of Australia on a private yacht bound for Greece.

After a massive manhunt he was arrested in Athens, wearing an ill-fitting wig, in June 2007 and extradited.

Tony Mokbel arrives at Royal Melbourne Hospital. Nine News

The maximum security prison near Lara, about 60 kilometres south-west of Melbourne, is home to more than 400 prisoners.

It is not the first time a high-profile criminal has been attacked at Barwon Prison. Gangland kingpin Carl Williams was beaten to death with the seat of an exercise bicycle in the prison's maximum-security Acacia unit in April 2010.

With Erin Pearson