More than 50 people were arrested in connection with a pro-cannabis rally in Hyde Park as the Metropolitan police executed its Total Policing strategy to clear away protesters who had shrouded Speakers’ Corner in sweet-smelling smoke.

The London 4/20 meet-up and protest was due to run until 5pm on Sunday, but by 4.45pm word was sweeping the crowd that the police were operating a “zero tolerance” policy. Officers began entering the crowd to warn people to put out their joints.

“All we are doing is explaining to people that the 4/20 demonstration is over. We want to start returning the park back to normal,” said a liaison officer. “As the evening goes on the tolerance of cannabis is going to be less and less.”

Behind the blue-coated liaison officers, a separate line of police in yellow jackets advanced and pushed the crowd towards Marble Arch. Many smokers quickly finished off the remains of their cannabis for fear they might be picked off by police as they made their way home.

For several hours beforehand, dope smokers had been able to light up with relative freedom – as long as they didn’t stray too close to police. On the London 4/20 Facebook page, people had earlier said they expected “safety in numbers” would protect them. A steward from Norml – which campaigns for reform of cannabis laws – was overheard telling one group to “keep it discreet and hopefully the police will leave you alone”.

But at least one man was seen being led away from the crowds by police, who were highly visible around the event. Paul Birch, the chair of the Cannabis is Safer than Alcohol party, told the Guardian he had heard reports that some people were being stopped by police and searched for drugs on the way to the demonstration.

On Sunday evening, police said there were 53 arrests, all for drug-related offences. 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.

Police signs in Hyde Park warn people attending the event. Photograph: Zuma/Rex Shutterstock

Stuart Harper of Norml UK, which planned the 4/20 event, said police were generally accepting of the protest, “but … some people simply allow their exuberance to overtake themselves and police have to act on what they see. Cannabis is unfortunately still illegal.”

The cloud of cannabis smoke from the rally could be smelled half a mile away from Sunday’s demonstration, where which thousands gathered to celebrate cannabis culture. A PA brought by the organisers for speeches and music did not work, but many people had brought their own speakers.

Derek Stratts, 27, was sharing bags of green skunk from a jar with his two friends. “Personally, I come here to smoke,” said Stratts, an 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.”

But the Met’s response may have poured cold water on Stratts’s hopes. The force said it had put extra officers in place for the event. “Anyone seen by officers openly smoking cannabis in Hyde Park or elsewhere in Westminster will be dealt with,” a Met spokesperson had warned.

Stratts’s fate was unknown. He had earlier told the Guardian: “We’re here to just enjoy the day and walk past a couple of police officers smoking a joint.”

A protester exhales smoke from a joint at the 4/20 event. Photograph: Zuma/Rex Shutterstock

Birch dismissed concerns the event had a negative impact on the fight for reform by showing the worst of cannabis culture. “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 earlier drawn the police’s attention.

“If people like us, serious political organisations, 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.”

Harper had said Norml was expecting about 10,000 people for the Hyde Park rally, making it the UK’s biggest 4/20 event. Despite chilly weather, numbers grew to near that figure by the event’s climax at 4.20pm – the internationally recognised time for the first spliff of the day. People cheered to celebrate the moment, but most of the mix of activists, hippies, and baggy-trousered youths had begun smoking long before that.

Pedro, 31, a banking and finance student, regarded the drug’s continuing prohibition as a question of hard-nosed economics. “It can be grown so you can’t tax it,” he said. On the subject of drug law reform, he added: “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.”

But there might not be. Recent research by YouGov found that roughly half (49%) of British adults are opposed to the legalisation of cannabis and a third (32%) are in favour, roughly the reverse of the situation in the US. Jon Liebling, the political director of the United Patients Alliance, which campaigns for medicinal cannabis, agreed there was still “a path to go down”.

“There’s very few people who object to patients being given cannabis as a medical thing and once they realise the world doesn’t fall to pieces I think the number will change very, very quickly,” he said.

Liebling said he self-medicated with cannabis to treat anxiety and depression, after bad experiences with Prozac. Now, he said, he faces trial on 1 June for production of a controlled drug. “Which is growing my own medicine,” he said. “And I’m not going to apologise for it anymore.”