By Janet Guttsman

TORONTO (Reuters) - There is a moment in the Russian mob movie "Eastern Promises" when the level of violence rises so high that the audience lets out a collective gasp, followed by a ripple of nervous laughter.

But director David Cronenberg and his star Viggo Mortensen insist the vicious climax to a murderous bathhouse battle between mob killers is an essential part of the movie, bringing home the reality and the finality of death.

"Murder is a serious thing. I am taking it very seriously," Cronenberg told Reuters in an interview on the sidelines of the Toronto International Film Festival, where "Eastern Promises" had its premiere on Saturday night.

"I'm an atheist," Cronenberg said. "To me an act of murder is the act of total destruction, it's absolute. There's no comeback, there's no going to heaven, that's it. And it is very easy for that to be veiled or covered up, in a movie especially.

"To me it makes perfect legitimate, artistic and, if you push me, moral sense as well to do that this way."

The movie pairs Cronenberg with Mortensen for the second time in three years after the two worked together in the Oscar-nominated "A History of Violence," another movie about crime and how people respond to it.

Mortensen, speaking a convincing Russian-accented English, plays a chillingly efficient driver for a Russian crime syndicate in a grimy, rain-swept London, although there is of course more to driver Nikolai than first meets the eye.

"I worked really hard," Mortensen said of his efforts to perfect a Russian accent and to learn to speak the jargon that a gangster might use.

HEAD-TO-TOE TATTOOS

His movie tattoos, the head-to-toe signature marks of a criminal who served time in a Russian jail, were so convincing that he twice frightened Russians in London before deciding it was best to scrub them off after a day on the set.

The making of the movie coincided with the real-life murder of former Russian spy Alexander Litvinenko, who died in London in November 2006 after receiving a dose of radioactive polonium-210.

"The Litvinenko poisoning was while we were filming," Cronenberg said, reminiscing about haz-mat suits and forensic vans outside a building near where the crew was working. "Sure enough they found traces of polonium there. We are undoubtedly totally polluted."

The movie opens in Russia this week but Cronenberg said feedback was already positive.

"We hear the Russian criminals are loving the movie because of the accuracy," he said.

"The moral aspect of it is not really the issue for them. The issue is are we being mocked and did we get it right? Or did we get it wrong? And so far we have passed. They are not worried about being shown being criminals because they are, so why should they be upset about it?"