Police arrested 12 people as thousands gathered in Hyde Park to mark the capital’s annual demonstration calling for the drug to be decriminalised.
Plumes of smoke could be seen rising above large crowds in the central London park during this year’s 420 protest.
Campaigners say they believe attitudes towards the Class B drug have changed and hope it could become legal in the near future.
A large police presence was seen throughout the demonstration, and random police checks were carried out as the force issued a warning to drug users.
Twelve people were arrested at the demonstration by 5.30pm over drugs-related offences, the Met Police said.
But protestors say a large police presence was a "waste of time", and that it was a peaceful protest.
Sending police to hyde Park today is a waste of resources😂 #420— D. (@Djblue96D) April 20, 2017
Sy Dignam, 43, of the Hampshire Cannabis Community, said: "I was here every year for the last four years. Four years ago there was 4,000 and I'm sure there's many more here now.
“We've all had enough - the cannabis community is massive. There are nine events like this across the country.
A reminder for those attending the rally at #HydePark today, officers will be enforcing drugs legislation #420— MPS Events (@MetPoliceEvents) April 20, 2017
“It's never escalated. We're such peaceful protesters.
"I've never seen trouble in four years of doing it."
At Thursday’s rally, Paul Roberts, 46, of the Tottenham Compassion Club, told of the supposed positives of using the drug.
He said: "We're here to try and raise awareness of what you can do with cannabis medically.
“It should be allowed for everyone. The way we look at it - in any community there are different aspects of doing it. We've chosen this path."
Lucy Pearce, 19, of Vester Avenue, in St Albans, said she has smoked since she was 14.
She said: "I couldn't say I've ever had any bad experiences with it.
"If you look it up on the internet there are so many cases with epilepsy were it has helped.
“I feel like [these events] do put some pressure but it just seems like they don't want to do much about it."
The origin of 420 is subject to numerous urban myths and theories.
It has been suggested that the number 420 came from the penal code the Californian police used to categorise cannabis cases.
In 1971, students at a Californian high school allegedly organised to meeting at 4.20pm to find a plot of land to plant a cannabis plant.
Rallies and celebrations of weed are set to take place across the world today, stretching as far as New Zealand and America.