Police arrested 53 people as pro-cannabis protestors gathered to get high and call for the drug to be legalised in London's Hyde Park today. Dramatic images captured the moment four officers took down and restrained a tracksuit-wearing man near the entrance to the park - just yards from a prominent police sign which read: 'Possession of cannabis is illegal'. The crowds met this afternoon as part of the annual '420' event - a global celebration of cannabis which calls for reform to drug laws. Scroll down for video . Taking him down: The four police officers were seen rushing towards the man at the entrance to Hyde Park . Restrained: The pro-cannabis protestor is pushed down to the ground just yards from a police sign reading: 'Possession of cannabis is illegal' Thousands of campaigners were seen sitting on the grass openly smoking marijuana, comparing the size of their joints and listening to speakers while dozens of police officers hovered nearby. Many protestors handed out cannabis leaf-shaped fliers and carried signs, with one reading: 'Ed Miliband wouldn't want his own children criminalised for cannabis possession.' Of the 53 who were arrested in London, 16 were taken into custody, while 21 were released on street bail and asked to return to a station at a later date. A Met Police spokesman said: 'Anyone seen by an officer openly smoking cannabis in Hyde Park or in the Westminster area could be issued with a warning or face arrest.' The term '420' has become universally known as the code word for cannabis - after a group of  students coined it while searching for an abandoned crop of marijuana in California in 1971. A man wearing a President Obama mask holds a marijuana joint to his mouth in Hyde Park today . Campaigners were seen  openly smoking cannabis while dozens of police officers stood nearby . The festival is usually held on  April 20  and last year there were estimated to be around 10,000 people openly smoking marijuana in Hyde Park. The 420 Day event is organised by a reform-seeking group called NORML UK, along with the UK Cannabis Social Clubs (UKCSC). Last year, Greg de Hoedt, president of UKCSC, told the Guardian last year: 'The cannabis community is crying out for legalization and regulation. 'We encourage growers to register with UKCSC because we want data we can present to authorities and say: this many people grow cannabis, this many people are growing for medicinal purposes; and we can also establish which strains work with which illnesses.' Four people hold their marijuana joints together with a bag full of the drug visible in the centre of the photo . Many campaigners carried signs including this message referring to Labour Leader Ed Miliband . This weekend also saw campaigners gather in Glasgow's George Square and in Manchester's Platt Fields Park. The Manchester event's Facebook page reads: 'This will be our fifth annual gathering so we plan to make it one to remember. 'For those who haven't been there's going to be good tunes, good food, good company, games and activities, face painting, freebies, talks stalls and information all in the name of celebrating the many aspects of cannabis culture.' Checking for drugs: Police officers stop and search a visitor to Hyde Park this afternoon . The 420 Day event is organised by a reform-seeking group called NORML UK, along with the UK Cannabis Social Clubs (UKCSC) A protestor blows smoke from a marijuana joint during taday's pro-cannabis festival in Hyde Park . More than 50 people were arrested at today's event in London, with 16 taken into police custody . The controversial pro-cannabis event is set to continue into tomorrow with a 'Parliament Protest Picnic', according to the 'London 420' Facebook page . This weekend also saw campaigners gather in Glasgow's George Square and in Manchester's Platt Fields Park .
Thousands met in Hyde Park today as part of annual global '420' event .
Crowds seen openly smoking marijuana as police officers stood nearby .
Controversial festival  also saw gatherings in Manchester and Glasgow .