Police dogs were kept busy by the thousands who streamed into Hyde Park for the annual 420 cannabis picnic on Wednesday, where 20 people were arrested for a range of drugs offences and a dozen more issued with cannabis warnings.

The retrievers employed by the Metropolitan police to sniff out illegal substances were stationed by the royal park’s Marble Arch entrance, just a few hundred yards from where a crowd that eventually numbered in the thousands smoked spliffs and ate cannabis edibles in an act of mass civil disobedience.

The picnic – held every year on or around 20 April – defied its midweek scheduling by drawing more than 5,000 enthusiasts. At 4.20pm, the internationally recognised time for lighting up a joint, a cheer – and an extra-thick cloud of smoke – went up.

It was a moment of togetherness for dope smokers. And it must have felt like Christmas for the sniffer dogs, who were rewarded with treats for each suspect they greeted at the police checkpoint set up to snare unsuspecting stoners.

I watched This dog catch two people with drugs in five minutes in Hyde park #London420 pic.twitter.com/H7IT9vLfH3 — Damien Gayle (@damiengayle) April 20, 2016

By the end of the day, one 27-year-old woman had been arrested on suspicion of possession with intent to supply a class A substance, 19 people were arrested for possession of cannabis and 12 more were issued with cannabis warnings, according to the Met.

Three more people were arrested at the event on suspicion of robbery.

The drug arrests and warnings came despite the organisers claiming they had negotiated a “safe conduct zone” – where smoking weed discreetly would be more or less tolerated – with the Met before the event.

Police liaison officers patrolling the picnic denied any such arrangement, but conceded there would be a “proportionate response” to the protest. “We will be leaning more towards cannabis warnings,” said one, “but obviously if we see a high level of criminality – people dealing, smoking bongs – then we will act. There are children around in the park.”

“Cannabis is still illegal,” her colleague interjected.

By 4pm, just before the climax of the day, the crowds milled together. Picnickers stood or lounged in clumps, passing joints or puffing vaporisers. Portable soundsystems played reggae and drum’n’bass. Hawkers sold cannabis-themed T-shirts and drummers beat out a rhythm.

The two kinds of signs at #420 in London. pic.twitter.com/beSVE10eXm — Damien Gayle (@damiengayle) April 20, 2016

One of the drummers, Ant Hatcher, with the group Original African Indianz, did not care whether or not cannabis was legalised. “Whether it is or not, it’s always going to be here and people love it,” he said.

“I smoke herb and I mix it with peppermint sometimes. I’m a naturalist, I believe that nature heals. I don’t believe in pharmaceuticals. The herb is a healer. I don’t just smoke it, I eat it as well and I eat the seeds, I drink the powder. I use the oil, I put it on my skin, use it on my hair.”

Pulling up his trouser leg, he added: “My socks are made out of cannabis – these are hemp socks!”

Next to a bright green Brighton cannabis club flag sat Rob Davidson, who said he was there with friends “to celebrate our love of cannabis, and take a stand against unjust prohibition laws”.

Davidson, the chairman of the club, described it as a local organisation that attempted to find a community-led solution to cannabis prohibition. “We don’t want a capitalist takeover of weed,” he said. “We hold private events, which are ticketed, friendly events. We don’t sell or distribute cannabis at the events, but we allow people to come and smoke.”

Rob Davidson, chairman of the Brighton cannabis club, said he was at the 420 picnic to ‘take a stand against unjust prohibition laws’. Photograph: Vianney Le Caer/Rex/Shutterstock

But a capitalist takeover of weed seems to be in the pipeline. Stuart Harper, of Norml UK, the self-styled political wing of the cannabis movement, was upbeat about it. “There’s millions flooding in at the moment,” he said. “There’s rich Mexicans and rich Americans over here with millions in investment capital. In the next 18 months there’s going to be a big shift.”

The business possibilities of the “Big Dope” lobby are serious news. Investment firms such as Privateer Holdings are investing in brands including Marley Natural and Leafly and are lobbying governments around the world. That same capitalist enthusiasm is bleeding down to the grassroots.

Junior, 26, a Slovakian, was at his first 420 protest. “If they would legalise it I would like to immediately start a business,” he said. “There are thousands of people who are into weed and you can find here people from each area of life.”

Bryannie-Laura, 24, from Bedford, said cannabis should be legalised for the good of mankind. “I think it’s really important that we legalise cannabis for its medicinal purposes,” she said. “Some of the stuff that’s happened in America is life changing. It can cure cancer.

“It’s time for change, it’s time to legalise it. It grows in the ground. It’s like magic mushrooms: it makes no sense. I mean, it’s a natural thing.

“Look at the number of people smoking. It’s going to happen, you can’t squash it. It’s a movement. And the government would make a lot more money if they legalised it – everyone can be a winner.”

A man wears a ‘Don’t get caught’ hat during the rally. Photograph: Vianney Le Caer/Rex/Shutterstock

On the way out, by the pelican crossing over Park Lane, a man with a white goatee and an American accent looked over with a smile. “Nothing ever changes,” he sighed.

What do you mean? “You know what that’s about right?” he said. “I was out there in ’78, doing the exact same thing. Nearly got arrested.”

But that was decades ago – times have changed. Do you think they can win now?

He laughed. “No.”