Johnny Depp's portrayal of James 'Whitey' Bulger was tipped Friday as a potential Oscar winner after "Black Mass", an account of the Boston mobster's life of crime was unveiled to a warm reception at the Venice film festival.

Greeted by thousands of fans, some of whom had been camped out all night, the American star kicked off an at-times rambling press conference with a joke acknowledging that he may appear to be slightly the worse for wear.

"This is non-alcoholic," he said after sipping from a water bottle. "I'm being responsible. So if I slur it's your fault," he said.

Barely recognisable after having been given the Irish-American gangster's severely receding hairline, sallow Celtic complexion and piercing blue eyes, Depp features in almost every scene of Scott Cooper's movie.

It is a biopic that could almost be classed as a buddy movie since at its core is the relationship between Bulger and John Connolly, a childhood friend from the tough, Irish-dominated area of South Boston, who has become an FBI agent.

Connolly persuades his bosses that he can bring down the Italian mafia in the city, if they offer Bulger a degree of protection as an incentive to help provide the leads and evidence they need.

The Faustian pact initially yields rewards, allowing Connolly's career to blossom in parallel with Bulger's development of an empire that grows to eventually include illicit gambling, extortion rackets and much of the city's drug trade.

But as it becomes clear that Connolly has little, if any, control of his supposed informant, Bulger's eventual downfall becomes only a matter of time.

The film ends with Connolly's arrest on charges which eventually lead to him being jailed for life for corruption and for knowingly causing the death of one of Bulger's many victims.

Bulger became a fugitive in 1994 and evaded capture for 17 years. For much of that time he was classed as second only to Osama bin Laden as the United States' most wanted man.

He was finally captured in 2011. Two years later he was convicted of involvement in 11 murders and sentenced to spend the rest of his lift in prison, where he turned 86 on Thursday.

A complex character

Cooper's account spares viewers the worst incidents of extraordinary brutality that punctuated Bulger's criminal career while still providing ample testimony of the sociopathic, sadistic core of his personality.

In one scene culled from real life, he strangles a friend's girlfriend to death in front of him after discovering she knew more than she should about their affairs.

But the Bulger portrayed by Cooper is also a man bestowed with a natural charisma who lets his elderly mother cheat at cards - a world away from the Bulger-based monster played by Jack Nicholson in Martin Scorsese's "The Departed."

Without overlabouring the point, the film includes a reference to the fact that Bulger had taken LSD repeatedly for experiments conducted during an earlier stint in prison.

It also depicts how badly Bulger was affected by the sudden death, from Reye's syndrome, of his six-year-old son, overtly suggesting the loss helped to explain what followed.

"I just had to approach him as a human being," Depp said. "Nobody wakes up and looks in the mirror and thinks 'I'm evil, today I'm going to do something evil.'

"One side to Bulger was his business and in that business he did what he had to do. But there was also a side to him that was a very loving family man. He was a very complicated man."

Cooper revealed that Bulger's lawyer, on a visit to the set, had described Depp's incarnation of his client as "uncanny and chilling," and some influential critics were also impressed.

Variety called his "mesmerising" performance as the best of his career and hailed the film as "one of the fall's first serious awards-caliber attractions."

The Hollywood reporter was not so gushing about the production, branding it "derivative" in places because of its many nods to classic gangster films "The Godfather" and "Goodfellas," but concurred that Depp's performance had been "one of his best."

Depp has been nominated for a Best Actor Oscar three times before but has never won.