Many of this year's runners got cold feet over the threat of legal action

A "zany" annual Halloween tradition has failed to materialise in Boulder, Colorado, with the threat of police action quashing the Nude Pumpkin Run.

Each year, dozens have run down the city's streets wearing only shoes and a hollowed-out pumpkin on their heads.

Police said "full" participants this year faced indecent exposure charges.

Up to 100 officers were deployed around Boulder this Halloween. As of early Sunday, local media reports said only one Halloween-related arrest was made.

"I have heard people say, 'It's just not worth it. Let's not go down [to the city's mall]'," Boulder's Police Chief Mark Beckner told local newspaper the Daily Camera.

'Legal threat'

Local reports say with up to 4,000 people taking part in Halloween revelry, a handful of individuals did take part in the run, but that when police caught up with them they were sufficiently clothed as to be "within the law".

A Boulder police spokesperson said as of early morning Sunday only one Halloween-related arrest had been made, which was not connected to the Naked Pumpkin Run.

A charge of indecent exposure could have led to participants being registered as sexual offenders.

Last year, some 150 people turned out naked for the 10th event of its kind.

Similar runs have also taken place in other US cities including Seattle, Portland and Arcata.

On its website, the group had warned potential runners that "the violation of the Western societal more, enforced by law, of unclothed public exposure can indeed land you legal consequences.

"Furthermore, the decision to participate is yours and yours alone," it added.