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More than 50 people were arrested on suspicion of drugs-related offences during a "pro-cannabis" rally in Hyde Park today.

Drugs campaigners gathered at the "4/20 Day Celebration" to hear speeches and smoke the drug openly - despite police signs warning "possession of cannabis is illegal".

Thirty-seven people were taken to Hyde Park police station, which does not have a custody suite, while 16 were taken to police stations elsewhere in the capital and detained.

Despite the arrests, organiser Stewart Harper of Norml-UK said the demonstration had been peaceful.

It was one of 420 arranged internationally to argue that "we need legal access for cannabis and medical access to cannabis especially," he added.

"I can completely understand [worries about addiction and other health problems caused by the drug] - but if you regulated the market to make sure substances were not sold to children and that it was regulated then that is a better situation, rather than leaving it without control and to anarchy in the black market.

"We prefer to look at this as a health issue rather than a criminal issue. We are thinking of how we can reduce the harm rather than saying 'this is illegal'. Drugs [are] a real issue around the world."

He called for a rethink of how the drug is treated in Britain and also pointed towards large scale problems of drugs smuggling and people who are killed or kidnapped internationally as a reason for changes to be made.

At the #420 demo at #HydePark this afternoon as part of the team for Grassroots: The Cannabis Revolution. #itwasforwork #honestly A photo posted by Andrew Jones (@bonesii) on Apr 19, 2015 at 10:26am PDT

Of the arrests, 10 people were given cannabis warnings, six were issued with fixed penalty notices, and 21 were given street bail and told to attend police stations on future dates. Sixteen were arrested and held on suspicion of drugs-related offences.

A similar event two years ago dubbed "National Pot Smoking Day" saw just two people arrested.

Facebook page the London Cannabis Club called the event "another successful friendly rally".

But some criticised the Met for arresting the alleged drug-users. "If only they would do the same to the politicians and bankers destroying the world rather than a few smokers enjoying the world," said Phil Hicks.

Additional reporting by PA