A PR prank to test how people would react if they witnessed a murder in progress has provoked fierce debate.

In a two-minute video posted online, a number of unsuspecting bystanders are confronted with the sight of a man trying to kill another man on the floor of a lift in a New York apartment block.

The clip, entitled Elevator Murder Experiment, is a publicity stunt for the upcoming gangland thriller Dead Man Down, starring Colin Farrell.

The video captures some witnesses attempting to help the victim while others stand, watch and run away, and one man even records the 'murder' on his smartphone.

When the clip was online it asked viewers the question: "What would you do if you walked in on a murder in progress?"

"Two guys asked themselves the same question and staged a murder-in-progress inside a New York City elevator. Hidden cameras recorded people's reactions. What would you do?" the description box reads.

The viral footage, which has already garnered thousands of hits on YouTube, has stirred up a huge response from the online community.

Many viewers argued that it was a creative way to address the controversial 'bystander effect.'

One viewer said: "If i was there, if it was a guy like him, i would probably help, but if it was a bulky guy, waaayyy stronger than me.... i really do not know.

Another wrote: "I would have helped. Do for others what you hope they may do for you. You would want help, so help others."