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A protest against Government plans to ban legal highs saw dozens of people stage a "mass inhalation" of laughing gas outside Parliament today.

The demonstration in Parliament Square saw protesters fill balloons with nitrous oxide - often called laughing gas or "hippy crack" - before inhaling from them all at the same time at 3pm.

The organisers were hoping hundreds would attend - after more than 1,500 signed up to a Facebook event - but only about 60 or 70 people took part.

The event was organised by The Psychedelic Society, which is campaigning against the government's proposed Psychoactive Substances Bill.

Laughing gas is one of a number of so-called legal highs which would be outlawed.

WATCH: Protestors in Parlt Sq inhale laughing gas en masse, against govt plans to ban sale of legal highs. @LBC pic.twitter.com/MCF2ZInfA6 — Simon Conway (@SimonConway) August 1, 2015

Writing on the Facebook page, the society said: "Only a few substances would be exempt: alcohol, nicotine and caffeine.

"This is ridiculous. We should be free to buy, sell and use whatever substances we want, so long as we do so responsibly and without harming others.

"These are our bodies, our minds, and it should be up to us what we do with them."

The society has adopted a twitter hashtag of #mymindmychoice to promote its views.

The Government's proposed law would ban any mood-altering drugs that are not specifically exempt under current legislation.

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."