Cannabis smokers have congregated in London's Hyde Park for an annual gathering which calls for the drug to be decriminalised.

Thousands of enthusiasts are expected to assemble inside the capital's largest park for the 420 event, which is described as a "significant" celebration of cannabis and its culture

Photographs captured on Thursday show smokers lighting up on the grass, some armed with drug paraphernalia, as police warned they would intervene in illegal behaviour.

Anyone found guilty of possessing cannabis, which is classified as a Class B drug, can be sentenced to five years in prison and a maximum term of 14 years for dealing or producing the illegal drug.

The event is held every year on April 20 which references the American English system of ordering dates, as 20 April would be stylised in the US as 4/20.

Ten people were arrested at last year's event for a range of drug and violent offences.

The Metropolitan Police said they would be patrolling the park this year to intervene in any illegal activity and signs have been erected to remind the public not to indulge in recreational drugs in the public space.

A police spokesman said: "The Met will be policing the cannabis day event in Hyde Park on Thursday, 20 April.

"There is a proportionate and graduated policing plan in place to prevent, deter and detect criminal offences, including possession and use of drugs, and any associated crime or disorder.

The statement continued: "Using cannabis and other recreational drugs is illegal. Met officers will engage with attendees, and encourage them to deposit their drugs in the amnesty bins provided.

"They will particularly be looking to safeguard any children and young people present, especially if they are using drugs, and will ensure they are referred to the relevant partners.'

"Officers may enforce legislation under the Misuse of Drugs act 1971 through Community Resolution Orders, Postal Charge Requisitions and arrest if there are aggravating factors such as supply or associated anti-social behaviour, disorder or crime."

The website Hemp Elf said the gathering in London was expected to be "significant"

It reads: 'The term 420 has become a symbol of unity and celebration within the cannabis community. It represents a shared understanding and passion for the plant and its potential benefits.

The origins of 420 are said to lie in California in the 1970s when a group of friends from San Rafael high school met after school at 4.20pm before attempting to locate a marijuana crop at Point Reyes, north-west of San Francisco.

The group began using the term 420 before it spread with help from members of the Grateful Dead rock band.