Cannes (France) (AFP) - Hollywood actor Joaquin Phoenix wowed Cannes Saturday as a hammer-wielding hitman tasked with saving a young girl from an elite prostitution ring, in a thriller he said had a feminist twist.

"You Were Never Really Here" by British director Lynne Ramsay set the competition for the Palme d'Or top prize alight just before awards night on Sunday, after a 70th edition short on big hits.

In a year that has seen stars such as Nicole Kidman and Salma Hayek hit out at show business sexism, Ramsay is one of only three women among the 19 film-makers vying for glory at the world's biggest film festival.

Critics said Ramsay had a solid chance of becoming its second female director to win Cannes, after Jane Campion for "The Piano" in 1993.

In the film's 85 minutes of intense violence and psychodrama, Phoenix plays Joe, an Iraq veteran and former FBI agent who is hired by a New York state senator to rescue his daughter from a paedophile sex ring catering to politicians.

The mission dredges Joe's own traumas to the surface including an abusive father and atrocities he witnessed as a soldier. He resorts to self-harm including suffocating himself with towels and plastic bags to shut out the memories.

The "Gladiator" and "Her" star said Joe, who uses a hammer as his weapon of choice, explored the "impotence of masculinity".

"We wanted to get away from that idea of the male hero," he said.

"I think what's maybe interesting about this film... is that really the girl is ultimately the one that saves herself."

- 'It's a traumatising time' -

Phoenix bulked up for the role of Joe, a middle-aged former Marine who is out of shape but still has his killer instincts.

"We very much wanted to stay away from the kind of typical physical body that you find with these kind of characters...."

"We wanted him to be big as possible, (have) this kind of armour."

Phoenix said much of the script was improvised on the set of the film, which was financed in part by Amazon and whose score was written by Jonny Greenwood of Radiohead.

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Ramsay, who was last in competition at Cannes in 2011 with "We Need to Talk About Kevin" starring Tilda Swinton, said her highly-strung film mirrored a deep sense of anxiety in today's tumultuous world.

"It's a sort of traumatising time at the moment and I just like to explore characters, like their flaws in their beauty," she said.

The movie left many critics enraptured, drawing comparisons with the Martin Scorsese 1976 classic "Taxi Driver".

Tim Robey of the Daily Telegraph gave it five out of five stars. "The immensity of Ramsay's film lies in the scalpel surgery of her image-making, distilling and triple-distilling the stuff she shot to make every second count."

"Ramsay has made something extraordinary," said Jessica Kiang of entertainment website The Playlist, "a film that's both cruel and compassionate, composed of quick, stabbing slivers of insight about how childhood terror can be twisted up with adult compulsion."