On a cool spring afternoon, on the edge of a crowd numbering several thousand in a London park, Derek Stratts, 27, shared bags of cannabis from a jar with his two friends.

“Personally, I come here to smoke,” said Stratts, a trained accountant who asked for his real name not to be used. “I’m not, I’ll be real with you, really here for the movement, I’m more for being able to actually come out and to Hyde Park, of all places, and smoke a spliff with no aggravation.”

Yet amid the clouds of pungent smoke surrounding Speaker’s Corner on Sunday, at least one man was led away by police, to a chorus of jeers from the thousands of cannabis smokers attending the annual 4/20 gathering. The event, named after the American date convention, is an annual global festival of cannabis on or near 20 April.

“Last year 100 countries took part in 4/20 celebrations, this year it’s supposed to be 126,” said Stuart Harper of Norml UK, a pro-legalization group that hosts the event. “The celebration we have here in Hyde Park, which is about 10,000 people, is the largest in the UK.”

Those attending were a mix of activists, hippies and baggy-trousered young people. Pedro, 31, a banking and finance student, said he did not see the point of cannabis prohibition.

“The government are not taking the time to listen to what people want; whether or not it’s a majority or a minority, there’s enough people there,” he said, adding that the continuing prohibition was a question of hard-nosed economics. “It can be grown so you can’t tax it.”

4/20 Day has become an annual highlight of the campaign to legalize the drug despite complaints that its in-your-face smoking was damaging the cause of reform by showing the worst of cannabis culture. But Paul Birch, chair of the political party Cannabis Is Safer Than Alcohol (Cista), dismisses the criticism.

“You can’t stop events like today, it’s a global movement. That ship has sailed,” he said, as he stood near a stall advertising “illegal medical cannabis” that had already seen some attention from police.

“If people like us, serious political organizations, feel it’s OK to turn up, you can ask that question of course. But we feel it’s right to be here and obviously there’s a large number of people here who are going to be supportive of our party so it would be crazy not to be here.”

If 4/20 Day is a celebration of the recreational use of cannabis, it is at odds with the strategy being used to achieve an end to the drug’s prohibition. Cista, Normland others campaign most forcefully for legalization on medical grounds.

“I was once prescribed Prozac, which very nearly killed me and was probably the most scary experience of my life,” said Jon Liebling, political director of the United Patients Alliance, who self-medicates with cannabis for anxiety and depression. “Another doctor recommended I go back to using cannabis, because I know it helps.”

The police were generally accepting of the event, Harper said, “but generally speaking, some people simply allow their exuberance to overtake themselves and police have to act on what they see. If they see someone consuming cannabis then obviously they have to act. Cannabis is unfortunately still illegal.”

On Sunday evening, police said there were 53 arrests, all for drug-related offenses. Scotland Yard said 37 people were dealt with at Hyde Park police station, 10 were given a cannabis warning, six were handed a penalty notice, 21 were given police bail and 16 were taken into custody.

Stratts, however, was unconcerned at the prospect of arrest. “We’re here to just enjoy the day and walk past a couple of police officers smoking a joint,” he said.