Denver has become the first US city to decriminalise psilocybin, the psychoactive ingredient in magic mushrooms, following a public vote.

Decriminalisation led by a slim 51%, according to preliminary figures on Tuesday’s election released by Denver’s Election Division.

As many as 1,300 votes still remain to be counted, but that figure was not enough to swing the vote the other way, division spokesman Alton Dillard said.

Final election results will be released on May 16, he said.

“I think today’s outcome really demonstrates that the conversation is going to continue, and the world is ready for it,” said Cindy Sovine, chief political strategist for the campaign to decriminalise the drug.

“Psychedelics are already here. Now we can start to have the conversation about using them mindfully,” she added.

Organisers turned to the same strategy that marijuana activists used to decriminalise possession of the drug in the city in 2005.

That move was followed by statewide legalisation in 2012, and a number of other states have since broadly allowed marijuana sales and use by adults.

Campaigners said their only goal in the mushroom measure is to keep people out of jail in Denver for using or possessing the drug to cope with depression, anxiety, post-traumatic stress and other conditions.

“We’re not talking about legalisation, we’re talking about not putting people in jail,” Ms Sovine said.

The initiative effectively decriminalises use or possession of psilocybin by people 21 and older, making it the lowest enforcement priority for police and prosecutors.

It does not legalise psilocybin or permit its sale by cannabis businesses.

Psilocybin has been outlawed since the 1960s, when it was widely known as a recreational drug.

The ban stymied medical research, but small studies in recent years have found the substance had positive effects on anxiety and depression for cancer patients.

Users have described seeing vivid colours and geometric patterns and experiencing powerful spiritual connections and emotions.

Magic mushrooms have been used in religious practices for decades because of their powerful effect on perceptions and spiritual experiences.

Those same effects have appealed to recreational users dating to the 1960s counterculture movement.

Denver Mayor Michael Hancock and District Attorney Beth McCann opposed the initiative, but there was no organised campaign against decriminalisation.