On Easter Sunday, thousands of cannabis smokers will descend on Hyde Park in London for a smoke in celebration of "420 Day".

Now held annually on 20 April, 420 Day began life as an American pro-cannabis movement, but in recent years has become a de facto "world pot-smoking day".

Last year, according to the event's organisers, around 10,000 people congregated in the most famous of London's royal parks to protest in favour of legalisation. With a bumper bank holiday weekend, a delightful spring and 420 Day chatter spreading like wildfire across social networks, they expect the attendance this year to be even higher.

For the cannabis community, it is the highlight of the year, a day when Britain's 60-plus cannabis social clubs encourage members, friends and the curious to stand up and be counted. Or at least to "sit around, have a few joints, a picnic and a discussion", as Greg de Hoedt, president of UK Cannabis Social Clubs (UKCSC), describes the plan.

Centred on Speaker's Corner, 420 Day is an exercise in civil disobedience, rather than an authorised protest. It begins at noon, with live music and speakers, before the high point – in both senses – at 4.20pm, when thousands will light up simultaneously and send a plume of sickly sweet smoke wafting across Hyde Park and Park Lane.

For many, it is an alternative day out. "I don't smoke any more as I'm a dad," says 43-year-old first-time attendee Johnnie, from Crystal Palace. "But I am pro-legalisation, so I'm going to show my support."

The 420 Day event is hosted by NORML UK – an organisation providing a voice for responsible cannabis users in Britain – along with UKCSC and the London cannabis social club. The British cannabis lobby is well organised; medicinal and recreational users, producers and entrepreneurs plan campaigns and gather evidence and support through social media. "The cannabis community is crying out for legalisation and regulation," explains de Hoedt. "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."

Britain's pro-cannabis movement has been galvanised by major gains in the US, where 20 states allow marijuana for medical use, and, as of this year, Colorado and Washington permit sales for recreational use.

The cultivation, sale and consumption of cannabis is now legal in Uruguay, while Portugal and Canada have taken more progressive approaches to regulation. De Hoedt is optimistic that something similar will happen here: "We have 60,000 subscribers to cannabis social clubs. We came together as UK Cannabis Social Clubs because we share the same goals. We can see what's happening around the world and want to make that happen here."

So far, police have employed a tolerant approach. "Last year, there were lots of police but they didn't do anything," says de Hoedt. "Can they really arrest 10,000 people?"