Demonstrators have protested against Government drug policy by staging a mass inhalation of so-called "hippy crack" in the shadow of Parliament.
Dozens of people sat down in Parliament Square in Westminster where they filled balloons with nitrous oxide and simultaneously breathed in the "laughing gas".
The legal drug has the common effect of inducing laughter and the group cheered after staging their protest.
The Government plans to introduce a new law to ban any mood-altering drugs that are not specifically exempt under current legislation in a bid to tackle legal highs.
Authorities including police have issued regular warnings against the harmful use of legal highs, including laughing gas which has become a common sight at festivals.
Stephen Reid, founder of the Psychedelic Society, said: "The Government 's rationale for prohibiting trade in psychoactive substances is that their use can be risky.
"Yes, the use of psychoactive substances can be risky, but it should be for individuals to decide whether or not to take the risk.
"People should be able to buy, sell and use whatever substances they want, so long as there's no harm to others."
He likened using the drug to having a cigarette, describing it as a brief high which lasts up to 40 seconds.
"The fact is most people enjoy using drugs, whether coffee, alcohol or nitrous oxide and that careful legal regulation is the fairest and safest solution."
The Psychoactive Substances Bill, which follows a similar move in Ireland, introduces a blanket ban on the production, distribution, sale and supply of legal highs after they were linked to scores of deaths.
More than 500 new drugs have been banned by the Government but the current system is seen as laborious as substances have to be assessed individually before they can be outlawed and manufacturers often produce new versions almost immediately after a previous form has been prohibited.