Thousands of people are expected to gather in the central London park to smoke weed and to call on the Government to legalise the Class B drug.
The rally takes place every year on April 20. Today’s event puts even further demand on the Met Police which has officers on duty at climate protests across London organised by the Extinction Rebellion group.
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 a meeting at 4.20pm to find a plot of land to plant a cannabis plant.
Rallies and celebrations of weed were taking place across the world today, stretching as far as New Zealand and America.
Cannabis users can go to jail if caught, though traffickers and dealers are more likely to receive prison sentences.